diff --git a/CGAL_Core/include/CGAL/CORE/extLong.h b/CGAL_Core/include/CGAL/CORE/extLong.h
index 095c5105b8d..1447842b61e 100644
--- a/CGAL_Core/include/CGAL/CORE/extLong.h
+++ b/CGAL_Core/include/CGAL/CORE/extLong.h
@@ -31,6 +31,9 @@
 #include <CGAL/CORE/Impl.h>
 #include <CGAL/CORE/CoreAux.h>
 
+#include <limits>
+#include <type_traits>
+
 namespace CORE {
 
 #ifndef LONG_MAX
@@ -77,6 +80,9 @@ public:
   extLong(long);
   /// constructor for \c unsigned long
   extLong(unsigned long);
+  /// constructor for \c std::size_t
+  template<typename = std::enable_if_t<!std::is_same_v<unsigned long, std::size_t>>>
+  extLong(std::size_t s);
   //@}
 
   /// \name Arithmetic and assignment operators
@@ -188,6 +194,17 @@ inline extLong::extLong(unsigned long u) {
   }
 }
 
+template <typename>
+inline extLong::extLong(std::size_t u) {
+  if (u >= (std::numeric_limits<std::size_t>::max)()) {
+    val  = EXTLONG_MAX;
+    flag = 1;
+  } else {
+    val = static_cast<long>(u);
+    flag = 0;
+  }
+}
+
 // isNaN defaults to false
 inline extLong::extLong(bool isNaN) : val(0), flag(0) {
   if (isNaN) {