diff --git a/Number_types/include/CGAL/boost_mp.h b/Number_types/include/CGAL/boost_mp.h
index 38ff8b20ddb..4c598778222 100644
--- a/Number_types/include/CGAL/boost_mp.h
+++ b/Number_types/include/CGAL/boost_mp.h
@@ -224,9 +224,44 @@ struct RET_boost_mp <NT, boost::mpl::int_<boost::multiprecision::number_kind_rat
     : RET_boost_mp_base <NT> {};
 
 #ifdef CGAL_USE_MPFR
-// Because of this full specialization, things get instantiated more eagerly. Make it artificially partial if necessary.
+// Because of these full specializations, things get instantiated more eagerly. Make them artificially partial if necessary.
 template <>
-struct RET_boost_mp <boost::multiprecision::mpq_rational, boost::mpl::int_<boost::multiprecision::number_kind_rational> >
+struct RET_boost_mp <boost::multiprecision::mpz_int>
+    : RET_boost_mp_base <boost::multiprecision::mpz_int> {
+    typedef boost::multiprecision::mpz_int Type;
+    struct To_interval
+        : public CGAL::cpp98::unary_function< Type, std::pair< double, double > > {
+        std::pair<double, double>
+        operator()(const Type& x) const {
+#if MPFR_VERSION_MAJOR >= 3
+	  MPFR_DECL_INIT (y, 53); /* Assume IEEE-754 */
+	  int r = mpfr_set_z (y, x.backend().data(), MPFR_RNDA);
+	  double i = mpfr_get_d (y, MPFR_RNDA); /* EXACT but can overflow */
+	  if (r == 0 && is_finite (i))
+	    return std::pair<double, double>(i, i);
+	  else
+	  {
+	    double s = nextafter (i, 0);
+	    if (i < 0)
+	      return std::pair<double, double>(i, s);
+	    else
+	      return std::pair<double, double>(s, i);
+	  }
+#else
+	  mpfr_t y;
+	  mpfr_init2 (y, 53); /* Assume IEEE-754 */
+	  mpfr_set_z (y, x.backend().data(), GMP_RNDD);
+	  double i = mpfr_get_d (y, GMP_RNDD); /* EXACT but can overflow */
+	  mpfr_set_z (y, x.backend().data(), GMP_RNDU);
+	  double s = mpfr_get_d (y, GMP_RNDU); /* EXACT but can overflow */
+	  mpfr_clear (y);
+	  return std::pair<double, double>(i, s);
+#endif
+        }
+    };
+};
+template <>
+struct RET_boost_mp <boost::multiprecision::mpq_rational>
     : RET_boost_mp_base <boost::multiprecision::mpq_rational> {
     typedef boost::multiprecision::mpq_rational Type;
     struct To_interval