FLORENCE, Italy, Sept 15 (Reuters) - A European Union naval force deployed in the Mediterranean should turn back migrant boats after they leave Libya and prevent them from reaching Italy, British Foreign Minister Boris Johnson said on Thursday.

Italy is on the frontline of Europe's migrant crisis, taking in more than 400,000 refugees over the past three years, many of them saved from rickety boats pushed out to sea by people smugglers based in north Africa.

The European Union launched Operation Sophia in 2015 in response to the crisis, with a mandate to disrupt the people trafficking networks and destroy smugglers' boats.

Johnson said part of the mission's work was to return boats back to shore after they had put to sea.

"I think personally (the boats) should be turned back as close to the shore as possible so they don't reach the Italian mainland and that there is more of a deterrent," Johnson said, speaking alongside his Italian counterpart Paolo Gentiloni.

"I think I am right in saying we have turned back about 200,000 migrants," Johnson said, before a nearby diplomat hastily corrected him. "Sorry, saved, saved. Thank you. We have saved 200,000 migrants and turned back 240 boats."

It is illegal to turn back migrant boats once they reach international waters and a U.N.-backed government in Libya has not invited European ships into its territorial waters, saying this would undermine its own state-building efforts.

It was not immediately clear in what context the boats mentioned by Johnson were turned back to land. There was no immediate comment from Operation Sophia officials in Rome.

Johnson was in Italy for talks about Britain's decision to abandon the European Union.

(Reporting by Crispian Balmer; Editing by Keith Weir)