Meanwhile David Cameron declared himself "delighted" at the paper's shift back to the Conservatives, which threatened to overshadow the fightback manifesto launched yesterday 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.
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.
"In every country the Government has had to step in," he said.
"We started from a lower level of debt than other countries. As we accumulate the deficits that are necessary to pay for the recession, our debt levels still are lower than Germany and France and America, so it is wrong to say we have a unique problem."
Mr Brown dismissed the idea that he might means-test child benefit to save cash and denied plans for a pay freeze in the public sector, although he made clear that job losses could not be ruled out.
The support of The Sun is a much-coveted political prize. Tony Blair made it a priority to woo proprietor Rupert Murdoch as he looked to topple the Conservatives in 1997 and the paper famously claimed after Labour's 1992 defeat that "It's The Sun Wot Won It".
Political editor George Pascoe-Watson said the tabloid had warned Labour at the time of the 2005 election that it was on its "last chance".
"We feel now after four years that they have failed the country and they are letting Sun readers down," he said.
But former Labour communications chief Alastair Campbell played down the significance of the move.
"If Labour lose, it will not be The Sun wot lost it," he wrote on his blog.
"The switch was entirely predictable, and had been evident for some time. It was merely a matter of when.
"For Labour, it should actually help the feeling of fighting back that has finally been around this week. If you're the underdog, might as well have it clear that you're the underdog."
Joint leader of the Unite union Tony Woodley described The Sun's intervention as "a cynical attempt to do the most damage to Gordon Brown on the very day that he enjoyed a bounce-back in the polls and delivered such an important speech to conference."
He added: "I am not surprised, because while The Sun was prepared to support Tony Blair's Labour, which was about promoting the free market and big business, we now have a Prime Minister who has gone back to our values. The leopard spots have returned - The Sun has never supported any of the progressive policies achieved by Labour, such as the minimum wage."
And former deputy prime minister John Prescott sent a defiant message via Twitter: "It will be the Son, Daughter, Uncle, Mother and Friend Wot Win it in 2010. Endorsements from ordinary people NOT media barons."
Labour Deputy Leader Harriet Harman used an appearance on the conference platform to attack The Sun.
She said: "I am speaking to you this morning about something The Sun knows absolutely nothing about: equality.
"The nearest their political analysis gets to women's rights is Page 3's News in Briefs.
"We are all angry about The Sun this morning but I say to you: don't get bitter, get better.
"Don't get outraged, get out there. Don't get mad, get mobilised.
"Yes, we may be the underdog but we will not be bullied. This underdog is biting back."
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