Two US tourists who chipped off a piece of the Colosseum in Rome 25 years ago have returned it - along with an apology for taking it.

The fragment of stone, small enough to fit into a pocket, arrived in Italy in a package from California.

A note inside read: "We should have done this sooner."

Rome's archaeology officials have accepted the couple's apology and the local tourism officer has invited them to return to the city.