Liberal Democrat leader signalled that he would speak to the Tories first about the formation of a minority government

Nick Clegg today signalled that he would speak to the Conservatives first about the formation of a minority government if Labour came third by share of the vote on 6 May, rejecting the constitutional convention that the prime minister should be allowed to try to form a government first.

The Liberal Democrat leader also made it explicit for the first time that electoral reform would be an unavoidable precondition of any coalition government as he insisted that Labour will have forfeited the right to govern if it comes third.

Labour tonight claimed Clegg had blundered by prematurely setting out new, detailed conditions on what would happen if there is no clear winner in the general election – claiming he looked arrogant and self-interested.

Lord Mandelson, Labour's election strategist, immediately warned in a campaign memo that "voters who flirt with Nick Clegg are likely to end up married to David Cameron". He said Clegg "had made clear his hostility to Labour and his preference to side with the Tories in a coalition if this arises. In other words, vote Nick and get Dave and George – not a nice prospect for people with progressive values."

The latest tracking poll from YouGov in the Sun suggests that Labour remains in third place, but the Conservatives are not drawing away from Liberal Democrats. The poll shows the Conservatives on 34 points (no change), the Liberal Democrats on 30 (up 1) and Labour on 28 (down 1).

The Lib Dems insisted that Clegg's remarks were being over-interpreted, and he was merely rejecting the constitutional assumption that the prime minister in the event of a hung parliament would always have the first opportunity to try to form a minority government.

Clegg said he would not prop up Labour if it came third in the vote yet secured the most seats. He said: "It seems to me that it's just preposterous, the idea that if a party comes third in terms of the number of votes, it still has somehow the right to carry on squatting in No 10 and continue to lay claim to having the prime minister of the country.

"What I'm saying here is pointing at a very, very irrational possible outcome of our potty electoral system, which is that a party that has spectacularly lost the election because fewer people are voting for it than any other party, could nonetheless according to constitutional tradition and convention still lay claim to providing the prime minister of the country."

With the campaign entering its final full week, Clegg may feel he needed to send out an anti-Brown message as polling suggested the Tory leader, David Cameron, was gaining traction with his warning that if voters back Clegg, they will end up with Brown in No 10.

Clegg also spoke for the first time about the possibility of sitting in cabinet with rival parties after the general election.

Liberal Democrat officials insisted their leader was not demanding Brown's head as the price of co-operating with Labour in the event of a hung parliament. But Clegg said Cameron and Brown would be vulnerable within their parties if they failed to secure an overall majority.

With characteristic frankness, the home secretary, Alan Johnson, said: "Look, it doesn't take a genius to think that if you are third in the popular vote, then … you are not best-placed to deal with it. We want to be first in the popular vote. We are going out over the next 11 days – and there are an awful lot of uncommitted voters."

Johnson said he was open to backing a more proportional voting system, closer to what Clegg wants, but another Labour electoral reformer, Peter Hain, told the Guardian that proportional systems break the link with constituencies and so make it more difficult to sack corrupt MPs.

Mandelson claimed in his campaign letter "as much as a third of the electorate are undecided. The challenges facing the country require a workhorse at the helm, not a couple of show ponies."

Liberal Democrats and Conservatives today ridiculed a request by Labour to broadcasters to focus more on policy analysis. The opposition parties claimed the appeal revealed Labour's unease at Brown's weak performances in the leadership debates.

Brown will try to cut through what he regards as the media's refusal to focus on policy by making three set-piece speeches this week on health, families and crime.

His speeches will put more emphasis on fairness and equality, rather than discussions on if it is right to withdraw £6bn from the economy this year. Appearing without a tie, and offering more pugilistic rhetoric than before, he said: "The Tory motto is not 'God helps people who help themselves', but 'God helps those whom he has already helped'."