The leaders of some of Britain's largest trade unions on Monday made a joint appeal to their 6 million members not to vote to leave the European Union, warning that a British exit would lead to the erosion of workers' rights.

"After much debate and deliberation we believe that the social and cultural benefits of remaining in the EU far outweigh any advantages of leaving," the leaders of Unison, the GMB, Unite and seven other unions said in a letter published in The Guardian.

Prime Minister David Cameron and senior figures from three other political parties also issued a joint statement on Monday, accusing the Vote Leave campiagn of "perpetuating an economic con-trick on the British people" by failing to give a detailed economic plan for Britain outside the EU.

"It's time for the Leave campaign to outline their economic plan for Britain outside Europe," Cameron, former acting Labour leader Harriet Harman, Liberal Democrat leader Tim Farron and Green Party leader Natalie Bennett said at a Remain campaign event in London.

Cameron also criticised Vote Leave's "simply wrong" claim that Britain could be forced to contribute to bailouts of eurozone nations.

"We are not part of eurozone bailout schemes," he said. "We also have a veto over any EU budget increases."

Vote Leave was "undemocratic and reckless" by failing to outline its economic plans, Cameron said.

Most opinion polls put the Remain vote marginally ahead, but many people are still undecided and there is uncertainty about how turnout could affect the result.

In a Daily Telegraph poll of some 19,000 subscribers, 69 per cent of respondents said they plan to vote for a British EU exit, or Brexit, in the June 23 referendum.

The Telegraph readers made Cameron's fellow Conservative Boris Johnson, who heads the Vote Leave campaign, their favourite to become Britain's next prime minister.

The newspaper said Monday's comments by Cameron reflected the intensification of a "civil war" over Brexit in the Conservative Party.

The trade union leaders said British and European unions had "fought hard to secure valuable working rights legislation at EU level," including maternity and paternity rights, and equal treatment for full-time, part-time and agency workers.

"If Britain leaves the EU, we are in no doubt these protections would be under great threat," they said.

The EU's recent political direction "has taken governments down a path of austerity and liberalization," they said, promising to "help shape a Europe with a renewed social agenda and a Europe that values investment in our public services."

The Remain campaign is expected to continue to stress the economic costs of a British EU exit, or Brexit, while the Leave side is likely to increase its focus on immigration ahead of the referendum on June 23.

Cameron made the referendum a key promise in his manifesto ahead of a general election in May 2015, when the Conservatives won 330 of the 650 parliamentary seats.

He negotiated a deal earlier this year for EU reforms that would give Britain what he called a "special status."

But he is likely to come under pressure to step down if the public votes to leave the EU, and Johnson is widely seen as a leading contender to succeed him.

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