diff --git a/lang/cpp/src/key.cpp b/lang/cpp/src/key.cpp
index e536d080..f9cc2b60 100644
--- a/lang/cpp/src/key.cpp
+++ b/lang/cpp/src/key.cpp
@@ -866,6 +866,12 @@ UserID::Signature::Signature(const shared_gpgme_key_t &k, gpgme_user_id_t u, gpg
 }
 
 bool UserID::Signature::operator<(const Signature &other)
+{
+    // kept for binary compatibility
+    return static_cast<const UserID::Signature *>(this)->operator<(other);
+}
+
+bool UserID::Signature::operator<(const Signature &other) const
 {
     // based on cmp_signodes() in g10/keylist.c
 
diff --git a/lang/cpp/src/key.h b/lang/cpp/src/key.h
index a7931672..515bf185 100644
--- a/lang/cpp/src/key.h
+++ b/lang/cpp/src/key.h
@@ -468,7 +468,9 @@ public:
     }
 
     /*! Defines a canonical sort order for signatures of the same user ID. */
-    bool operator<(const Signature &other);
+    bool operator<(const Signature &other) const;
+
+    GPGMEPP_DEPRECATED bool operator<(const Signature &other);
 
     bool isNull() const
     {