The Cabinet has secretly discussed plans to boost the Liberal Democrats' chances of winning next month's by-election in Oldham East and Saddleworth. The Cabinet talks will infuriate Tory traditionalists, who suspect that David Cameron would be happy for the Liberal Democrats to win the contest.

Andrew Mitchell, the Conservative International Development Secretary, is believed to have called on his party to do everything possible to help the Liberal Democrats beat Labour in the 13 January contest.

The by-election was discussed when the Cabinet reviewed the Coalition’s strategy for 2011 on Tuesday, The Independent has learnt. There was a brief “political session”, without civil servants present, at the end of the Cabinet’s weekly meeting.

Cabinet sources say David Cameron and other ministers did not disagree with Mr Mitchell’s remarks. But some of his Tory colleagues were angry and surprised because – in public – the Tories insist they are fighting to win the by-election in what is seen as a three-way marginal.

The contest has been caused by the disqualification of Phil Woolas, Labour’s former Immigration Minister, who had a tiny majority of 103 over the Liberal Democrats at the May general election. The Tories were only 2,310 votes behind in third place. The result was declared void last month by an election court, which found that Mr Woolas had made false statements about Elwyn Watkins, the Liberal Democrat candidate.

The Cabinet talks will infuriate Tory traditionalists, who suspect that Mr Cameron would be happy for the Liberal Democrats to win the Oldham East contest. A week ago, he said he “wished them well” in the by-election but insisted the Tories were fighting to win it.

Labour hopes that disaffection amongst people who voted Liberal Democrat in May at Nick Clegg’s decision to join the Tories in coalition will outweigh any backlash against Mr Woolas. Some Tory MPs suspect Mr Cameron wants to save Mr Clegg from a by-election defeat that could erode support for the Coalition amongst Liberal Democrats.

The Cabinet’s discussion will also fuel speculation that Mr Cameron and Mr Clegg may seek to prolong the Coalition beyond the next general election. At a press conference on Tuesday, the Prime Minister left the door open to an electoral pact or anti-Labour tactical voting.

Yesterday Peter Lilley, the former Cabinet minister and a right-wing Eurosceptic, became the latest Tory to suggest the Coalition could be extended beyond 2015. He told BBC Radio 4: “I can conceive that we might fight [the next election] as a coalition. I think the process of being in government had produced a degree of realism, on the part of a lot of the Liberals that will make them better partners in future than they were a prospective partners when this Coalition started….We might find even people on my wing of the party will say perhaps better have them in bed with us than fighting us in the election.”

A continuing coalition has already been backed by Sir John Major, the former Prime Minister; Nicholas Boles, a Cameron ally and Tory MP and Jacob Rees-Mogg, a right-wing Tory backbencher.

But Peter Bone, another right-wing Tory MP, said the “vast bulk” of Conservatives want to fight the next election as a separate party. He hoped the prospect of a joint coalition ticket had been “killed off” by this week’s comments by several Liberal Democrat ministers to undercover reporters posing as constituents, in which they criticised some government policies, Mr Cameron and the Chancellor George Osborne.

As nominations closed in Oldham East yesterday, the Tories denied claims that they were soft-pedalling to help their Coalition partners.

Iain Wright; MP for Hartlepool and Labour’s campaign manager in the by-election: “I have seen no activity from the Tories whatsoever. I think they have left it too late.” But Kashif Ali, the Tory candidate, dismissed the suggestion “bonkers” and “preposterous” and said he was “very happy” with support he was getting from the Tories regionally and nationally.

A Tory spokesman said: “We would not comment on what may or may not have been said at Cabinet but Andrew [Mitchell] is right behind Kashif Ali’s campaign and has already confirmed that he will be in Oldham East & Saddleworth for the campaign early in the New Year.”

Mr Cameron, the Foreign Secretary William Hague, the Communities Secretary Eric Pickles and the Defence Secretary Liam Fox will also campaign in the constituency.