Tory ministers considering a tactical vote deal to stop Labour

A plan for the Conservatives and Liberal Democrats to encourage anti-Labour tactical voting at the next general election is being discussed privately by Tory Cabinet ministers.

Although David Cameron and Nick Clegg insist the two Coalition parties will fight the election as separate parties, some Tory ministers want their party to soft pedal in the seats held by the Liberal Democrats.

The move could throw a lifeline to the 57 Liberal Democrat MPs, including Mr Clegg, who may come under pressure in his Sheffield Hallam constituency after his marked U-turn over university tuition fees.

Ministers believe a formal electoral pact in which the Tories and Liberal Democrats stand down to give each other a clear run in some seats is highly unlikely, as it would run into strong opposition from local party activists. However, they think this month's by-election in Oldham East and Saddleworth shows that an informal pact could work to the advantage of both parties – and hurt Labour.

The Tories ran a low-key campaign and many of their natural supporters switched to the Liberal Democrats, ensuring that Mr Clegg's party came second behind Labour and avoided a humiliating third place that could have led to demands from their activists to pull out of the Coalition.

Supporters of an informal pact at the election due in 2015 believe it could save many Liberal Democrat seats and give Mr Cameron an insurance policy if he failed to win an overall majority. In those circumstances, they hope the Coalition would continue – a course already backed by the former Prime Minister Sir John Major and the Tory former Cabinet minister Peter Lilley.

One Tory Cabinet minister told The Independent yesterday: "One scenario is an informal pact in which the two Coalition partners would help each other where they could. Voters aren't stupid. They can work out who is best placed to win in their constituency – as they did in Oldham. It wouldn't have to be a formal, top-down pact, which would be very difficult to impose on local associations in any case."

The minister added: "Are we going to go hell for leather to defeat Chris Huhne [the Liberal Democrat Energy Secretary] in Eastleigh [where the Tories are in second place]? No. I wouldn't have any part in that. I am not going to campaign for a Tory victory there."

The Liberal Democrats are more cautious about a pact than the Tories, reflecting fears in Mr Clegg's party that it is in danger of losing its identity in the Coalition. Mr Clegg said on Sunday that the Coalition partners were not "joined at the hip" and would fight the next election as separate, independent parties.

Privately, senior Labour figures fear an anti-Labour understanding. One reason why Ed Miliband is wooing the Liberal Democrats is to keep open the prospect of a Lib-Lab deal in the event of another hung parliament.

The planned Lib-Con pact faces opposition from right-wing Tory MPs, who were furious at the party's "softly softly" campaign in the Oldham by-election. But one minister dismissed their criticism, saying: "What was in the best interests of the Conservative Party? For the Coalition to continue and not to be destabilised. In the case of Oldham, that meant the Lib Dems doing well."

Senior Tories who favour a continuing alliance with the Liberal Democrats believe a pact could be more formal if a switch to the alternative vote is approved in the referendum due on 5 May. Under this system, voters list the candidates in order of preference, the bottom one drops out and votes are redistributed until one candidate wins more than 50 per cent of those cast.