diff --git a/mlir/lib/Transforms/Utils/CFGToSCF.cpp b/mlir/lib/Transforms/Utils/CFGToSCF.cpp
index dbde75e17b3b8..1a3d12661e7e7 100644
--- a/mlir/lib/Transforms/Utils/CFGToSCF.cpp
+++ b/mlir/lib/Transforms/Utils/CFGToSCF.cpp
@@ -1156,8 +1156,12 @@ static FailureOr<SmallVector<Block *>> transformToStructuredCFBranches(
     FailureOr<Operation *> result = interface.createStructuredBranchRegionOp(
         opBuilder, regionEntry->getTerminator(),
         continuation->getArgumentTypes(), conditionalRegions);
-    if (failed(result))
+    if (failed(result)) {
+      Region &parentRegion = *regionEntry->getParent();
+      for (Region &reg : conditionalRegions)
+        parentRegion.getBlocks().splice(parentRegion.end(), reg.getBlocks());
       return failure();
+    }
     structuredCondOp = *result;
     regionEntry->getTerminator()->erase();
   }
diff --git a/mlir/test/Conversion/ControlFlowToSCF/unsupported-op-in-cycle.mlir b/mlir/test/Conversion/ControlFlowToSCF/unsupported-op-in-cycle.mlir
new file mode 100644
index 0000000000000..58bffdb569328
--- /dev/null
+++ b/mlir/test/Conversion/ControlFlowToSCF/unsupported-op-in-cycle.mlir
@@ -0,0 +1,24 @@
+// RUN: mlir-opt %s -lift-cf-to-scf -verify-diagnostics
+
+// verify faliure for unsupported ops in cycles (issue #173566)
+
+module {
+  func.func @spirv_loop_crash_repro(%arg0: index) {
+    %0 = builtin.unrealized_conversion_cast %arg0 : index to i32
+    %cst8 = spirv.Constant 8 : i32
+    
+    spirv.mlir.loop {
+      spirv.Branch ^bb1(%0 : i32)
+    ^bb1(%2: i32):
+      %3 = spirv.SLessThan %2, %cst8 : i32
+      // expected-error @+1 {{Cannot convert unknown control flow op to structured control flow}}
+      spirv.BranchConditional %3, ^bb2, ^bb3
+    ^bb2:
+      %4 = spirv.IAdd %2, %0 : i32
+      spirv.Branch ^bb1(%4 : i32)
+    ^bb3:
+      spirv.mlir.merge
+    }
+    spirv.Return
+  }
+}
\ No newline at end of file