diff --git a/icu4c/source/common/ucnv_bld.c b/icu4c/source/common/ucnv_bld.c
index 42db2467daa..062bd6d7c04 100644
--- a/icu4c/source/common/ucnv_bld.c
+++ b/icu4c/source/common/ucnv_bld.c
@@ -197,7 +197,7 @@ isCnvAcceptable(void *context,
  * Un flatten shared data from a UDATA..
  */
 static UConverterSharedData*
-ucnv_data_unFlattenClone(UDataMemory *pData, UErrorCode *status)
+ucnv_data_unFlattenClone(UConverterLoadArgs *pArgs, UDataMemory *pData, UErrorCode *status)
 {
     /* UDataInfo info; -- necessary only if some converters have different formatVersion */
     const uint8_t *raw = (const uint8_t *)udata_getMemory(pData);
@@ -254,7 +254,7 @@ ucnv_data_unFlattenClone(UDataMemory *pData, UErrorCode *status)
     data->dataMemory = (void*)pData; /* for future use */
 
     if(data->impl->load != NULL) {
-        data->impl->load(data, raw + source->structSize, status);
+        data->impl->load(data, pArgs, raw + source->structSize, status);
         if(U_FAILURE(*status)) {
             uprv_free(data->table);
             uprv_free(data);
@@ -268,7 +268,7 @@ ucnv_data_unFlattenClone(UDataMemory *pData, UErrorCode *status)
  *goes to disk and opens it.
  *allocates the memory and returns a new UConverter object
  */
-static UConverterSharedData *createConverterFromFile(const char* pkg, const char *fileName, UErrorCode * err)
+static UConverterSharedData *createConverterFromFile(UConverterLoadArgs *pArgs, UErrorCode * err)
 {
     UDataMemory *data;
     UConverterSharedData *sharedData;
@@ -277,13 +277,13 @@ static UConverterSharedData *createConverterFromFile(const char* pkg, const char
         return NULL;
     }
 
-    data = udata_openChoice(pkg, DATA_TYPE, fileName, isCnvAcceptable, NULL, err);
+    data = udata_openChoice(pArgs->pkg, DATA_TYPE, pArgs->name, isCnvAcceptable, NULL, err);
     if(U_FAILURE(*err))
     {
         return NULL;
     }
 
-    sharedData = ucnv_data_unFlattenClone(data, err);
+    sharedData = ucnv_data_unFlattenClone(pArgs, data, err);
     if(U_FAILURE(*err))
     {
         udata_close(data);
@@ -481,23 +481,23 @@ ucnv_deleteSharedConverterData(UConverterSharedData * deadSharedData)
  * If pkg==NULL, then this function must be called inside umtx_lock(&cnvCacheMutex).
  */
 UConverterSharedData *
-ucnv_load(const char *pkg, const char *realName, UErrorCode *err) {
+ucnv_load(UConverterLoadArgs *pArgs, UErrorCode *err) {
     UConverterSharedData *mySharedConverterData;
 
     if(err == NULL || U_FAILURE(*err)) {
         return NULL;
     }
 
-    if(pkg != NULL && *pkg != 0) {
+    if(pArgs->pkg != NULL && *pArgs->pkg != 0) {
         /* application-provided converters are not currently cached */
-        return createConverterFromFile(pkg, realName, err);
+        return createConverterFromFile(pArgs, err);
     }
 
-    mySharedConverterData = ucnv_getSharedConverterData(realName);
+    mySharedConverterData = ucnv_getSharedConverterData(pArgs->name);
     if (mySharedConverterData == NULL)
     {
         /*Not cached, we need to stream it in from file */
-        mySharedConverterData = createConverterFromFile(NULL, realName, err);
+        mySharedConverterData = createConverterFromFile(pArgs, err);
         if (U_FAILURE (*err) || (mySharedConverterData == NULL))
         {
             return NULL;
@@ -703,8 +703,16 @@ ucnv_createConverter(UConverter *myUConverter, const char *converterName, UError
         /*   converter data cache, and adding new entries to the cache      */
         /*   to prevent other threads from modifying the cache during the   */
         /*   process.                                                       */
+        UConverterLoadArgs args={ 0 };
+
+        args.size=sizeof(UConverterLoadArgs);
+        args.nestedLoads=1;
+        args.options=options;
+        args.pkg=NULL;
+        args.name=realName;
+
         umtx_lock(&cnvCacheMutex);
-        mySharedConverterData = ucnv_load(NULL, realName, err);
+        mySharedConverterData = ucnv_load(&args, err);
         umtx_unlock(&cnvCacheMutex);
         if (U_FAILURE (*err) || (mySharedConverterData == NULL))
         {
@@ -763,30 +771,36 @@ UConverter*
 ucnv_createConverterFromPackage(const char *packageName, const char *converterName, UErrorCode * err)
 {
     char cnvName[UCNV_MAX_CONVERTER_NAME_LENGTH], locale[ULOC_FULLNAME_CAPACITY];
-    uint32_t options=0;
     UConverter *myUConverter;
-    UConverterSharedData *mySharedConverterData = NULL;
+    UConverterSharedData *mySharedConverterData;
+
+    UConverterLoadArgs args={ 0 };
 
     if(U_FAILURE(*err)) {
         return NULL; 
     }
 
-    /* first, get the options out of the convertername string */
-    parseConverterOptions(converterName, cnvName, locale, &options, err);
+    args.size=sizeof(UConverterLoadArgs);
+    args.nestedLoads=1;
+    args.pkg=packageName;
+
+    /* first, get the options out of the converterName string */
+    parseConverterOptions(converterName, cnvName, locale, &args.options, err);
     if (U_FAILURE(*err)) {
         /* Very bad name used. */
         return NULL;
     }
+    args.name=cnvName;
     
     /* open the data, unflatten the shared structure */
-    mySharedConverterData = createConverterFromFile(packageName, cnvName, err);
+    mySharedConverterData = createConverterFromFile(&args, err);
     
     if (U_FAILURE(*err)) {
         return NULL; 
     }
 
     /* create the actual converter */
-    myUConverter = ucnv_createConverterFromSharedData(NULL, mySharedConverterData, cnvName, locale, options, err);
+    myUConverter = ucnv_createConverterFromSharedData(NULL, mySharedConverterData, cnvName, locale, args.options, err);
     
     if (U_FAILURE(*err)) {
         ucnv_close(myUConverter);
diff --git a/icu4c/source/common/ucnv_bld.h b/icu4c/source/common/ucnv_bld.h
index 3d2a4c66920..d53c2142884 100644
--- a/icu4c/source/common/ucnv_bld.h
+++ b/icu4c/source/common/ucnv_bld.h
@@ -232,7 +232,7 @@ UConverterDataLMBCS;
  * If pkg==NULL, then this function must be called inside umtx_lock(&cnvCacheMutex).
  */
 UConverterSharedData *
-ucnv_load(const char *pkg, const char *name, UErrorCode *err);
+ucnv_load(UConverterLoadArgs *pArgs, UErrorCode *err);
 
 /**
  * Unload a non-algorithmic converter.
diff --git a/icu4c/source/common/ucnv_cnv.h b/icu4c/source/common/ucnv_cnv.h
index 17ba5b495d7..62856bbfd56 100644
--- a/icu4c/source/common/ucnv_cnv.h
+++ b/icu4c/source/common/ucnv_cnv.h
@@ -41,7 +41,18 @@ typedef struct UConverterSharedData UConverterSharedData;
 
 /* function types for UConverterImpl ---------------------------------------- */
 
-typedef void (*UConverterLoad) (UConverterSharedData *sharedData, const uint8_t *raw, UErrorCode *pErrorCode);
+/* struct with arguments for UConverterLoad and ucnv_load() */
+typedef struct {
+    int32_t size;               /* sizeof(UConverterLoadArgs) */
+    int32_t nestedLoads;        /* count nested ucnv_load() calls */
+    int32_t reserved;           /* reserved - for good alignment of the pointers */
+    uint32_t options;
+    const char *pkg, *name;
+} UConverterLoadArgs;
+
+typedef void (*UConverterLoad) (UConverterSharedData *sharedData,
+                                UConverterLoadArgs *pArgs,
+                                const uint8_t *raw, UErrorCode *pErrorCode);
 typedef void (*UConverterUnload) (UConverterSharedData *sharedData);
 
 typedef void (*UConverterOpen) (UConverter *cnv, const char *name, const char *locale,uint32_t options, UErrorCode *pErrorCode);
diff --git a/icu4c/source/common/ucnvmbcs.c b/icu4c/source/common/ucnvmbcs.c
index 8fa5e3a3d6e..70d02a10a8f 100644
--- a/icu4c/source/common/ucnvmbcs.c
+++ b/icu4c/source/common/ucnvmbcs.c
@@ -863,6 +863,7 @@ _EBCDICSwapLFNL(UConverterSharedData *sharedData, UErrorCode *pErrorCode) {
 
 static void
 _MBCSLoad(UConverterSharedData *sharedData,
+          UConverterLoadArgs *pArgs,
           const uint8_t *raw,
           UErrorCode *pErrorCode) {
     UDataInfo info;
@@ -884,6 +885,7 @@ _MBCSLoad(UConverterSharedData *sharedData,
     }
 
     if(mbcsTable->outputType==MBCS_OUTPUT_EXT_ONLY) {
+        UConverterLoadArgs args={ 0 };
         UConverterSharedData *baseSharedData;
         const int32_t *extIndexes;
         const char *baseName;
@@ -895,17 +897,28 @@ _MBCSLoad(UConverterSharedData *sharedData,
             return;
         }
 
+        if(pArgs->nestedLoads!=1) {
+            /* an extension table must not be loaded as a base table */
+            *pErrorCode=U_INVALID_TABLE_FILE;
+            return;
+        }
+
         /* load the base table */
         baseName=(const char *)(header+1);
         if(0==uprv_strcmp(baseName, sharedData->staticData->name)) {
             /* forbid loading this same extension-only file */
-            /* TODO better prevention of loading another extension table */
             *pErrorCode=U_INVALID_TABLE_FORMAT;
             return;
         }
 
-        /* TODO pass package name, same as current converter (see ucnv_bld.c) and/or parse out of prefix of base name */
-        baseSharedData=ucnv_load(NULL, baseName, pErrorCode);
+        /* TODO parse package name out of the prefix of the base name in the extension .cnv file? */
+        args.size=sizeof(UConverterLoadArgs);
+        args.nestedLoads=2;
+        args.reserved=pArgs->reserved;
+        args.options=pArgs->options;
+        args.pkg=pArgs->pkg;
+        args.name=baseName;
+        baseSharedData=ucnv_load(&args, pErrorCode);
         if(U_FAILURE(*pErrorCode)) {
             return;
         }
diff --git a/icu4c/source/test/testdata/conversion.txt b/icu4c/source/test/testdata/conversion.txt
index 2d105402e3c..33211ef5490 100644
--- a/icu4c/source/test/testdata/conversion.txt
+++ b/icu4c/source/test/testdata/conversion.txt
@@ -177,6 +177,15 @@ conversion {
     fromUnicode {
       Headers { "charset", "unicode", "bytes", "offsets", "flush", "fallbacks", "errorCode", "callback", "invalidUChars" }
       Cases {
+        // extension in testdata
+        {
+          "*test4x",
+          "\u20ac\x09",
+          :bin{ 0009 },
+          :intvector{ 0, 1 },
+          :int{1}, :int{1}, "", "?", ""
+        }
+
         // DBCS-only extensions
         {
           "ibm-970",
diff --git a/icu4c/source/test/testdata/test4x.ucm b/icu4c/source/test/testdata/test4x.ucm
new file mode 100644
index 00000000000..c7692aaece8
--- /dev/null
+++ b/icu4c/source/test/testdata/test4x.ucm
@@ -0,0 +1,20 @@
+# *******************************************************************************
+# * Copyright (C) 2003, International Business Machines
+# * Corporation and others.  All Rights Reserved.
+# *******************************************************************************
+#
+# test4x.ucm
+#
+# Test file for MBCS conversion extension with four-byte codepage data.
+
+<code_set_name>     "test4x"
+<mb_cur_max>        4
+<mb_cur_min>        1
+<uconv_class>       "MBCS"
+
+# test loading an extension table from the testdata package
+<icu:base>          "test4"
+
+CHARMAP
+<U0009>     \x09 |0
+END CHARMAP
diff --git a/icu4c/source/test/testdata/testdata.mk b/icu4c/source/test/testdata/testdata.mk
index b28455a7894..ba35661aedd 100644
--- a/icu4c/source/test/testdata/testdata.mk
+++ b/icu4c/source/test/testdata/testdata.mk
@@ -14,7 +14,7 @@ TESTDT=$(TESTPKG)_
 ALL : "$(TESTDATAOUT)\testdata.dat" 
 	@echo Test data is built.
 
-"$(TESTDATAOUT)\testdata.dat" : "$(TESTDATABLD)\casing.res" "$(TESTDATABLD)\conversion.res" "$(TESTDATABLD)\mc.res" "$(TESTDATABLD)\root.res" "$(TESTDATABLD)\te.res" "$(TESTDATABLD)\te_IN.res" "$(TESTDATABLD)\testaliases.res" "$(TESTDATABLD)\testtypes.res" "$(TESTDATABLD)\testempty.res" "$(TESTDATABLD)\$(TESTDT)iscii.res" "$(TESTDATABLD)\$(TESTDT)idna_rules.res" "$(TESTDATABLD)\DataDrivenCollationTest.res" "$(TESTDATABLD)\$(TESTDT)test.icu" "$(TESTDATABLD)\$(TESTDT)testtable32.res" "$(TESTDATABLD)\$(TESTDT)test1.cnv" "$(TESTDATABLD)\$(TESTDT)test3.cnv" "$(TESTDATABLD)\$(TESTDT)test4.cnv" "$(TESTDATABLD)\$(TESTDT)ibm9027.cnv" "$(TESTDATABLD)\$(TESTDT)nfscsi.spp" "$(TESTDATABLD)\$(TESTDT)nfscss.spp" "$(TESTDATABLD)\$(TESTDT)nfscis.spp" "$(TESTDATABLD)\$(TESTDT)nfsmxs.spp" "$(TESTDATABLD)\$(TESTDT)nfsmxp.spp"
+"$(TESTDATAOUT)\testdata.dat" : "$(TESTDATABLD)\casing.res" "$(TESTDATABLD)\conversion.res" "$(TESTDATABLD)\mc.res" "$(TESTDATABLD)\root.res" "$(TESTDATABLD)\te.res" "$(TESTDATABLD)\te_IN.res" "$(TESTDATABLD)\testaliases.res" "$(TESTDATABLD)\testtypes.res" "$(TESTDATABLD)\testempty.res" "$(TESTDATABLD)\$(TESTDT)iscii.res" "$(TESTDATABLD)\$(TESTDT)idna_rules.res" "$(TESTDATABLD)\DataDrivenCollationTest.res" "$(TESTDATABLD)\$(TESTDT)test.icu" "$(TESTDATABLD)\$(TESTDT)testtable32.res" "$(TESTDATABLD)\$(TESTDT)test1.cnv" "$(TESTDATABLD)\$(TESTDT)test3.cnv" "$(TESTDATABLD)\$(TESTDT)test4.cnv" "$(TESTDATABLD)\$(TESTDT)test4x.cnv" "$(TESTDATABLD)\$(TESTDT)ibm9027.cnv" "$(TESTDATABLD)\$(TESTDT)nfscsi.spp" "$(TESTDATABLD)\$(TESTDT)nfscss.spp" "$(TESTDATABLD)\$(TESTDT)nfscis.spp" "$(TESTDATABLD)\$(TESTDT)nfsmxs.spp" "$(TESTDATABLD)\$(TESTDT)nfsmxp.spp"
 	@echo Building test data
 	@copy "$(TESTDATABLD)\$(TESTDT)te.res" "$(TESTDATAOUT)\$(TESTDT)nam.typ"
 	@"$(ICUP)\bin\pkgdata" -f -v -m common -c -p"$(TESTPKG)" -d "$(TESTDATAOUT)" -T "$(TESTDATABLD)" -s "$(TESTDATABLD)" <<
@@ -34,6 +34,7 @@ $(TESTDT)test.icu
 $(TESTDT)test1.cnv
 $(TESTDT)test3.cnv
 $(TESTDT)test4.cnv
+$(TESTDT)test4x.cnv
 $(TESTDT)ibm9027.cnv
 $(TESTDT)idna_rules.res
 $(TESTDT)nfscsi.spp
@@ -107,6 +108,9 @@ $(TESTDT)nfsmxp.spp
 "$(TESTDATABLD)\$(TESTDT)test4.cnv": "$(TESTDATA)\test4.ucm"
 	@"$(ICUTOOLS)\makeconv\$(CFG)\makeconv" -d"$(TESTDATABLD)" -p $(TESTPKG) $**
 
+"$(TESTDATABLD)\$(TESTDT)test4x.cnv": "$(TESTDATA)\test4x.ucm"
+	@"$(ICUTOOLS)\makeconv\$(CFG)\makeconv" -d"$(TESTDATABLD)" -p $(TESTPKG) $**
+
 "$(TESTDATABLD)\$(TESTDT)ibm9027.cnv": "$(TESTDATA)\ibm9027.ucm"
 	@"$(ICUTOOLS)\makeconv\$(CFG)\makeconv" -d"$(TESTDATABLD)" -p $(TESTPKG) $**