diff --git a/src/ooxml/java/org/apache/poi/openxml4j/opc/OPCPackage.java b/src/ooxml/java/org/apache/poi/openxml4j/opc/OPCPackage.java
index 203d5c18f2..ffd764918b 100644
--- a/src/ooxml/java/org/apache/poi/openxml4j/opc/OPCPackage.java
+++ b/src/ooxml/java/org/apache/poi/openxml4j/opc/OPCPackage.java
@@ -1664,6 +1664,16 @@ public abstract class OPCPackage implements RelationshipSource, Closeable {
         return partList.getUnusedPartIndex(nameTemplate);
     }
 
+	/**
+	 * @return true if the package is in Strict OOXML format
+	 * @since POI 5.0.1
+	 */
+    public boolean isStrictOoxmlFormat() {
+		PackageRelationshipCollection coreDocRelationships = getRelationshipsByType(
+				PackageRelationshipTypes.STRICT_CORE_DOCUMENT);
+		return coreDocRelationships.size() > 0;
+	}
+
 	/**
 	 * Has close been called already?
 	 */
diff --git a/src/ooxml/testcases/org/apache/poi/openxml4j/opc/TestPackage.java b/src/ooxml/testcases/org/apache/poi/openxml4j/opc/TestPackage.java
index 07e9a17d21..0d8b8360c8 100644
--- a/src/ooxml/testcases/org/apache/poi/openxml4j/opc/TestPackage.java
+++ b/src/ooxml/testcases/org/apache/poi/openxml4j/opc/TestPackage.java
@@ -103,6 +103,20 @@ public final class TestPackage {
 	private static final String CONTENT_EXT_PROPS = "application/vnd.openxmlformats-officedocument.extended-properties+xml";
 	private static final POIDataSamples xlsSamples = POIDataSamples.getSpreadSheetInstance();
 
+	@Test
+	void isStrictOoxmlFormat() throws IOException, InvalidFormatException {
+		try (OPCPackage p = OPCPackage.open(getSampleFileName("TestPackageCommon.docx"), PackageAccess.READ)) {
+			assertFalse(p.isStrictOoxmlFormat());
+		}
+		try (OPCPackage p = OPCPackage.open(xlsSamples.getFile("sample.xlsx"), PackageAccess.READ)) {
+			assertFalse(p.isStrictOoxmlFormat());
+		}
+		try (OPCPackage p = OPCPackage.open(xlsSamples.getFile("sample.strict.xlsx"), PackageAccess.READ)) {
+			assertTrue(p.isStrictOoxmlFormat());
+		}
+	}
+
+
 	/**
 	 * Test that just opening and closing the file doesn't alter the document.
 	 */