Tory MPs plot leadership challenge to Cameron David Cameron's leadership was under fresh pressure last night following claims that 14 Tory MPs have backed calls to remove him. The Spectator magazine reported that the group had written to Graham Brady, the chairman of the party's backbench 1922 Committee, asking for a leadership challenge. The number is well short of the 46 required to trigger a contest, but it is a new indication of the growing hostility to the Prime Minister among some of his own MPs. Some Tory critics believe he could be at his most vulnerable next year if there is little sign of economic recovery and the party is languishing in the polls. A Conservative MP recently revealed he had been approached by two backbenchers asking him to mount a "stalking horse" challenge to Mr Cameron, while speculation has intensified that Boris Johnson, the Mayor of London, is preparing the ground for a return to the Commons - and an eventual tilt at the leadership. Mr Brady refused to be drawn on the report, but said: "Nobody else apart from me would know whether it's true or not and I haven't spoken to anyone." Meanwhile, The Independent understands that the Prime Minister is about to signal a major retreat on plans to redraw Britain's parliamentary map by the 2015 general election, which would have handed a major advantage to the Conservatives. The moves to cut the number of seats from 650 to 600 are being opposed by the Liberal Democrats after the Tories pulled the plug on House of Lords reforms. Although Mr Cameron insisted last month that he was pressing ahead with the proposals, he is preparing to authorise Tory constituency parties in marginal seats to select candidates on existing, rather than proposed, boundaries. The announcement will be seen as a recognition that the Government will not be able to drive the proposals on to the statute book. A well-placed Tory source said: "It makes sense - Labour and the Lib Dems are starting to select on the old boundaries and we can't afford to get left behind." The latest monthly poll of polls for The Independent puts Tory support on 33 per cent in August (unchanged), eight points behind Labour on 41 per cent (down one). The Liberal Democrats are on 12 per cent (up one). John Curtice, a professor of politics at Strathclyde University, who compiled the "poll of polls," calculated that the figures would give Ed Miliband an overall majority of 90 on the old boundaries - but 76 if the next election is fought on the revised boundaries. Mr Cameron also faces a new challenge with the launch of a parliamentary campaign for a referendum on EU membership - a move opposed by the Tory leadership. Although it is aimed at MPs of all parties, it is likely to attract the highest levels of support from eurosceptics on the Conservative benches and could be a new focus for internal dissent. Its founder, the Tory MP John Baron said: "Our objective is to pressure the Government and the political establishment generally to allow the country its say as to our membership of the EU." Another rebellion is brewing against the Prime Minister over his commitment to same-sex marriages, with some Conservatives saying the measure has provoked more anger among Tory members than any other issue. Although there is little doubt that the planned legislation will be passed with Labour and Liberal Democrat support in a free vote, it is becoming a rallying point for Mr Cameron's internal enemies. One said: "No one can really understand why he has invested so much energy on this. There would be uproar in my constituency if I voted in favour.