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Gordon Brown has tried to brush off the impact of losing the support of The Sun, insisting that it is the British public, and not newspapers, who will decide the result of the coming general election.
Meanwhile David Cameron declared himself "delighted" at the paper's shift back to the Conservatives, which threatened to overshadow the fightback manifesto launched on Tuesday by the Prime Minister in his final conference speech before the election, expected in the spring.
In a round of TV interviews, the PM sought to turn attention back onto his new policies, which included free home care for elderly people, a maximum one-week wait for cancer tests, hostel accommodation for teenage mothers and a clampdown on drink-fuelled yobbery.
He also revealed that he has made his mind up whether to take on Mr Cameron and Liberal Democrat leader Nick Clegg in a US-style TV debate - but will not reveal his decision until nearer the election.
The Sun's announcement - under the banner headline "Labour's Lost It" - came just hours after Mr Brown left the stage in Brighton after delivering a make-or-break speech designed to energise supporters and improve Labour's dismal standing in the polls.
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Mr Brown tried to play down its impact: "It's the British people that decide the election, it's the British people's views that I am interested in... I think Sun readers actually, when they look at what I say, they will agree with what I said."
And his Cabinet lieutenant Lord Mandelson said: "I don't think the readers want The Sun to set on New Labour. The last thing Sun readers want is to see their newspaper turned into a Tory fanzine."
Mr Cameron welcomed the paper's support, but insisted it would not lead to him becoming complacent. "I think they have seen the Government is exhausted and out of ideas and they see a regenerated, refreshed Conservative Party ready to serve," he told LBC Radio.
The Tory leader dismissed Mr Brown's speech as "a long shopping list without many prices", accusing the PM of failing to address the need to reduce Government borrowing of £175 billion.
But Mr Brown insisted his plans were affordable by transferring money from lower priority areas of spending. He denied Tory claims that the record deficit run up in response to the recession put Britain in a uniquely perilous economic position.
