By Matthew West

Voters should support the Liberal Democrats in constituencies where Labour cannot win, Gordon Brown has indicated, further fuelling speculation that Labour expects to be unable to form a majority government after polling day.

In an exclusive interview with politics.co.uk the prime minister gave his strongest signal yet that voters should consider tactical voting in order to prevent the Conservatives forming the next government.

The prime minister was asked whether he endorsed an appeal made by Peter Mandelson in March to voters in the south-west of England. He suggested they should vote Liberal Democrat in order to stop Conservative candidates from winning seats in the region, as first reported by the Western Morning News.

"The Liberal Democrats have more in common with Labour than with any other party," Lord Mandelson added.

When asked for his views on the comments, the prime minister told politics.co.uk : "I don't think we want a Conservative government, let's be honest about that.

"But the choice is between a Labour government and a Conservative government. You're not going to end up with one of the minor parties running the government - no matter how keen you are to see their interests represented and no matter how interesting some of their policies may be.

"So I think what he [Lord Mandelson] is saying is if you don't want a Conservative government make sure you don't get one.

"I want everybody to vote Labour and I want people to vote for our party, and I want our vote to be the highest and I want our number of seats to be the highest.

"But if people don't want a Conservative government then they must make sure they don't allow the Conservatives in."

The comments go even further than those made by the transport secretary Lord Adonis, who raised the sensitive issue of tactical voting in a provocative article in the Independent on Friday morning.

In an article for the newspaper he wrote: "To avoid a Tory government after May 6th, it is vital to grasp now the fundamental Labour-Lib Dem identity of interest. This can best be served by Labour coming out of the election as strong as possible, able to form a government.

Lord Adonis added: "In Labour-Tory marginals, a vote for the Lib Dems is a vote which helps the Tories against progressive policies. And in Labour-Lib Dem marginals every Labour MP returned is a seat in the Commons more likely to put Labour ahead of the Tories and therefore better placed to form a government."

The comments are in direct contrast to the sentiments of Tony Blair before the 1997 general election.

During the 1997 campaign, activists who called for voters to vote Lib Dem to get the Tories out of power were expelled for breaking Labour party rules.

The Lib Dems have reacted furiously at being called a minor party as well as the suggestion that a vote for the Liberal Democrats would lead to another five years of a Gordon Brown-led Labour government.

The Tories are targeting a number of potentially vulnerable seats in the south-west including Somerton and Frome, Bristol North West and Taunton Deane, where the election contest is a straight fight between the Conservative and Lib Dems.

Lib Dem home affairs spokesman and former leadership hopeful Chris Huhne's seat in Eastleigh, where he is defending a majority of just 530, is also highly vulnerable. He is number 11 on the Tories' overall target list.

The Lib Dems are also weakened in Cornwall. Two seats with radical boundary changes in central Cornwall, Truro & Falmouth and St Austell and Newquay, will hit long-time incumbent Matthew Taylor's vote, while Colin Breed is standing down in Cornwall South East.

A senior Lib Dem source said: "As the Independent's front cover on Friday shows, Labour are sending out mixed messages on this all the time.

"Brown is more mealy mouthed than some of his colleagues. But I think people will see it as the partisan posturing it really is.

"We're not going to give a running commentary on the latest desperate tactic of Labour HQ, just keep campaigning for every vote for the Liberal Democrats!"