Mocked by colleagues, Cameron argues Britain is safer in EU

LONDON, May 9 (Reuters) - Britain needs the European Union to help fight Islamic State and rebuff a "newly belligerent" Russia, Prime Minister David Cameron said on Monday, making a "patriotic case" for membership that was immediately mocked by a one-time ally.

With just over six weeks to go before a referendum, both sides of the debate are sharpening their arguments, with Conservative former London mayor, Boris Johnson, attacking the prime minister for scaremongering by suggesting that Britain could trigger World War Three if it left the 28-member bloc.

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Cameron, setting out the security argument for Britain to vote to remain in the EU on June 23, drew on military history, invoking the memory of wartime leader Winston Churchill to bolster his case that "isolationism has never served this country well."

But it was his words that the EU had helped secure peace for 70 years that were seized upon by "Out" campaigners, who questioned whether he could seriously believe that a British exit, or Brexit, could see troops on the move in Europe again.

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"I want to show that if you love this country, if you want to keep it strong in the world and keep our people safe, our membership of the EU is one of the tools that helps us to do these things," Cameron told diplomats and campaigners.

"I'm not arguing that the EU alone has kept the peace in Europe these last 70 seventy years, of course not, NATO has played an absolutely key role ... (but) it's pretty extraordinary that countries that were fighting and killing each other are now finding a way to work together."

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He warned Britain that it could be exposed to greater threats if voters decided to leave the EU, underlining the role of security cooperation after dozens were killed in attacks by Islamic State in Brussels and Paris.

"We see a newly belligerent Russia, the rise of the Daesh (Islamic State) network to our east and to our south the migration crisis - dealing with these requires unity of purpose in the West," he said.

Brexit, he said, would "make cooperation more legally complex and make our access to vital information much slower."

JOHNSON ATTACKS

Johnson, whom the prime minister had once hoped to recruit to his "In" campaign, said the argument was "wholly bogus."

"I don't think the prime minister can seriously believe that leaving the EU would trigger war on the European continent, given that he was prepared only a few months ago to urge that people should vote leave if they failed to get a substantially reformed European Union," Johnson told campaigners and journalists.

He said Britain should pull out now to regain its sovereignty and democratic rights, leaving a failing organization.

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The former mayor, who is seen as a possible replacement for Cameron, called on Britons to demand answers from "In" campaigners, including over how high levels of immigration could be curbed when the country had no control of its borders.

Both campaigns are trying to find the arguments that resonate with British voters, who, according to opinion polls, are evenly split over which way to vote.

The "In" campaign, backed by Cameron, has so far mainly stuck to economic arguments, trying to deepen rifts in the "Out" campaign over what Britain would look like after Brexit.