LONDON Prime Minister David Cameron will try to sell his case for Britain staying in the European Union to parliament on Monday facing hostility from his own lawmakers and a plunging pound after London's mayor threw his support behind the exit campaign.
Boris Johnson, one of Britain's most popular politicians, announced on Sunday he supported leaving the bloc, dealing a blow to Cameron who vowed to campaign to stay in after striking a deal to reform Britain's ties with the EU last week.
The mayor's announcement, seen as increasing the chance of a British EU exit by giving the 'leave' campaign a much-needed figurehead, pushed sterling towards its biggest loss in almost six years against the dollar on Monday. Government bond prices also fell.
Cameron shrugged off Johnson's statement, saying the issue of EU membership cut through usual party political lines.
"The prime minister has been clear all along there will be different views on different sides of the argument," his spokeswoman said.
"Our message to everyone -- bearing in mind that it is people up and down the country that will have a vote in this referendum rather than just one individual -- is that we want Britain to have the best of both worlds."
Johnson, who has held his post as London mayor since 2008, defended his decision to go against his sometime ally, saying those who suggested Britain could not thrive outside the EU were the same people who wanted the country to make the "catastrophic mistake" of joining the euro single currency.
"There are people who don't think that Britain could stand on her very own two feet and all the rest of it. I have to say I think that is profoundly wrong," Johnson told the London Assembly, the elected body which holds the mayor to account.
His decision to lobby against Cameron was welcomed by leaders of the 'leave' campaign, which has been dogged by splits between factions and lacked a uniting political figure to spread its message that Britain needs to regain its sovereignty.
"I'm delighted that he's come out for leaving the European Union," former finance minister and chairman of the Vote Leave campaign Nigel Lawson told BBC radio. "He is a superb campaigner so he's a great asset."
Odds of a British exit rose to a 33 percent chance from about 29 percent after his announcement, according to bookmakers.
DE FACTO LEADER
While Cameron's most senior cabinet colleagues have stuck with him, six others have said they will campaign for an exit, highlighting the deep divide in his Conservative Party over Europe dating back to Margaret Thatcher.
The scale of divisions should become clear later on Monday when Cameron, who hailed Friday's agreement as handing Britain a special status in the bloc, is due to make a statement to lawmakers.
But while many Conservatives may have turned against the deal, the prime minister was due to receive a boost from business leaders who were set to sign a letter saying the country would be better off in the bloc. Several corporate sources told Reuters the letter would be published on Tuesday.
Many businesses are keen to end the uncertainty that has weighed on markets and companies scrambling to come up with a 'plan B' if Britain votes to leave the bloc -- a move that would transform the country's role in world trade.
Cameron's bid to remain in the EU also has the support of much of London's financial district as well as most of the Labour Party, major trade unions, international allies and Scottish nationalists.
Johnson, 51, a political showman whose light-hearted wit masks a fierce ambition, said on Sunday he did not want to go against the prime minister but believed the EU project was in danger of getting out of democratic control.
That led some commentators to question whether his stance is a designed to attract widespread eurosceptic support among Conservatives in a bid to succeed Cameron, who has said he will step down before the next election in 2020.
A British exit from the EU would rock the Union -- already shaken by differences over migration and the future of the euro zone -- by ripping away its second-largest economy, one of its top two military powers and by far its richest financial centre.
Pro-Europeans, including former Prime Ministers Tony Blair and John Major, have warned that an exit could also trigger the break-up of the United Kingdom by prompting another Scottish independence vote.
A poll published before Johnson's move showed the "in" campaign with a lead of 15 percentage points. However, polls have fluctuated widely and surveys suggest about a fifth of voters are undecided.
A third of voters said Johnson would be important in helping them decide which way to vote, an Ipsos MORI poll showed.
"The big battalions of the argument are unquestionably ranged against people like me: We are portrayed as crazy cranks and all the rest of it," Johnson said in announcing his position. "I don't mind, I happen to think that I'm right."
(Additional reporting by Guy Faulconbridge and Kate Holton, Writing by Guy Faulconbridge and Michael Holden, editing by Elizabeth Piper and Sonya Hepinstall)
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