David Cameron has been urged to rule out any electoral pact between the Conservatives and Liberal Democrats amid backbench Tory fears that ministers are plotting a "Frankenstein" merger between the two parties.
Senior Tory MP Mark Pritchard suggested that the "heart and soul of the Conservative Party" was under threat from ministers seeking a long-term settlement with the Lib Dems.
He called on the Prime Minister to make plain his own view of the coalition beyond 2015, when the next general election is due to be held. Mr Pritchard, secretary of the powerful Tory backbench 1922 Committee, warned there would be "fractures" if MPs and activists discovered that an electoral deal was being discussed "behind their backs".
A number of Tory MPs, as well as former prime minister Sir John Major, have spoken of the need for joint coalition candidates at the next election. There have been reports that a senior minister is also backing the idea. Mr Cameron has also fuelled suspicions that the Tories are not mounting a serious challenge in next week's Oldham East and Saddleworth by-election by wishing the Lib Dem candidate well.
At a joint press conference with Lib Dem Deputy Prime Minister Nick Clegg, the premier raised eyebrows by describing it only as a "likelihood" that their parties would campaign separately at the next general election.
Writing in the Mail on Sunday, Mr Pritchard called for "an early and unequivocal statement" that "no coalition agreement has been agreed, or is being proposed, beyond the current Parliament". "At the next general election, the Conservative Party must fight to win - to win an outright majority and send the Liberal Democrats packing," he said.
He said any attempts to forge a long-term alignment with the Lib Dems had to be discussed openly. "If the parliamentary and voluntary party discover that an electoral deal is being proposed behind their backs, then it is inevitable there will be political fallout, resulting in fractures the party can ill afford," Mr Pritchard wrote.
Meanwhile, in a sign of how seriously the Lib Dems are taking Oldham East and Saddleworth, it was announced that Mr Clegg will be visiting the constituency for the second time on Wednesday. Another six Lib Dem ministers will campaign there this week, as well as former leader Charles Kennedy, deputy leader Simon Hughes and party president Tim Farron.
Mr Cameron is yet to visit, although he has pledged to do so, when prime ministers have traditionally steered clear of by-election campaigns in the past.
Lib Dem candidate Elwyn Watkins - who came second to Labour's Phil Woolas by just 103 votes in last year's general election - said: "With just 103 votes in it there's only one way to beat Labour and that's to vote Lib Dem," he said.