LONDON (Reuters) - Former NATO secretary generals warned on Tuesday that a British exit from the European Union would help enemies of the West while ex-U.S. foreign and defense chiefs cautioned that Britain would have less clout outside the bloc.

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The double warning comes as the two campaigns for and against Brexit step up their rhetoric about the impact staying or leaving the EU would have on Britain’s security.

Prime Minister David Cameron said on Monday that Britain was safer in the EU while former London mayor Boris Johnson, a member of his Conservative Party, accused him of suggesting World War Three would break out should Britons vote to leave in a referendum on June 23.

The five ex-NATO chiefs - Peter Carrington, Javier Solana, George Robertson, Jaap de Hoop Scheffer and Anders Fogh Rasmussen - said the imposition of EU sanctions against Russia and Iran, a move led by Britain, showed the importance of the bloc.

“Brexit would undoubtedly lead to a loss of British influence, undermine NATO and give succor to the West’s enemies just when we need to stand should-to-shoulder across the Euro-Atlantic community against common threats,” they wrote in a letter to the Daily Telegraph newspaper.

In a separate letter to the Times, 13 former U.S. secretaries of state and defense and national security advisers from every U.S. administration from Barack Obama’s to Jimmy Carter’s in the 1970s said Britain’s global position would suffer if it left the EU.

“We are concerned that should the UK choose to leave the European Union, the UK’s place and influence in the world would be diminished and Europe would be dangerously weakened,” said the letter signed by, among others, former Secretaries of State George Shultz and Madeleine Albright.

Their warning echoes a similar message from Obama during the U.S. president’s visit to Britain last month.

Those campaigning for Brexit have repeatedly dismissed such warnings, saying membership of NATO, rather than the EU, was key to British security.

In a sign of deepening divisions within Cameron’s own party, Iain Duncan Smith, the former Work and Pensions Secretary, said Germany had sabotaged the prime minister’s plans to renegotiate Britain’s relationship with the EU, forcing him to drop his plans to demand an emergency brake on migration.

“They have a de facto veto over everything,” Duncan Smith told Tuesday’s Sun newspaper which accompanied their story with a picture of German Chancellor Angela Merkel holding a puppet Cameron.