Tactical voting could be key if Brown is to return to power
Gordon Brown has hinted that Labour supporters should vote Liberal Democrat in Lib Dem-Conservative marginals - but stopped short of saying it outright.
The Labour leader said: "If people don't want a Tory government, then they must make sure they don't allow the Conservatives to get in."
His comments follow Transport Secretary Lord Adonis's comments that Lib Dem voters should vote Labour in key seats.
Lib Dem leader Nick Clegg said it was a "naked" attempt to win votes.
Mr Brown made his comments in an interview with website politics.co.uk.
Lib Dems laugh off Adonis vote plea
"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 the highest, but if people don't want a Conservative government, then they must make sure they don't allow the Conservatives to get in," he said.
Lord Adonis earlier told the Independent that in tight contests between the Tories and Labour a vote for the Lib Dems helped "the Tories against progressive policies".
There was a "fundamental Labour-Lib Dem identity of interest", he said.
However, Nick Clegg disagreed, saying Labour was "no longer progressive".
BBC chief political correspondent Laura Kuenssberg said it was quite early in the election campaign for the parties to be talking about tactical voting - when people vote for one party in order to try to keep out another.
Tactical voting has been around for a long long time, there's nothing new in it... I would always advise people to vote Labour
Lord Adonis - now a Labour minister but formerly a Lib Dem councillor - said the Lib Dems had a national policy that was similar to Labour's, but only Labour could implement the programme.
"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."
He added that it was "nonsense" for the Lib Dem leader to pretend his party was equidistant policy-wise between the two larger parties.
The Lib Dems are the UK's third biggest party and have faced repeated questions about whether they would work with Labour or the Tories, in the event of a hung parliament.
Mr Clegg has said he will not be the "kingmaker" and has attacked the "cosy consensus" between Labour and the Conservatives, accusing them of blocking repeated attempts to reform politics on issues like expenses, the voting system and party funding.
He told the BBC Lord Adonis's article was a "naked and obvious attempt at trying to garner votes" lost by Labour over the past few years.
"If there's a commonality of interest it is between the two old parties who keep making the same old mistakes."
In his new year's message, Conservative leader David Cameron also attempted to reach out to Lib Dem voters, saying there was a "lot less disagreement than there used to be" between the two parties.
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