The Sun has supported Labour for the last 12 years.
Sky's political editor Adam Boulton described the move as a nail in the coffin of the Prime Minister.
Next the headline Labour's Lost It the paper's front page says:
"After 12 long years in power, the Government has lost its way.The Sun's political editor George Pascoe Watson speaking to Sky News said:
"Now it's lost The Sun's support too."
"We felt Labour had it within them to change the course of Britain's future.Watson said The Sun was 'impressed' by David Cameron, but promised:
"But we feel now that they have failed the country and are letting people down."
"We will always be critical friends of the Conservatives, as we were with Labour, we will tell them when they are going wrong."The paper has backed Labour since 1997 when Tony Blair led the party to victory in the General Election.
Justice Secretary Jack Straw said he 'regretted' The Sun's decision to turn away from Labour.
"Newspapers have influence, there is no question about it,
"We would rather have their support, but it does not undermine, however, the committment we have to the British people, nor the fact that I believe they have made a mistake on behalf of their readers.
"I think it is a mistake."
There is of course no hard evidence that newspaper endorsements affect their own readers, let alone the broader public, but they are significant in the battle of ideas.
The Sun newspaper's endorsement is probably the most significant of all.
UPDATE: 08:30 30/09/09
Mr Pickles on Sky News has said that The Sun's endorsement of the Conservative Party would not make them complacent, insisting they would have to work even harder to win the election.
"To get 117 seats [necessary for a Conservative majority] is a big ask, so for all Conservatives around here this endorsement means were going to have to work even harder.
"It's now up to us to persuade the public that we can form the government and can be trusted with the economy.
"Complacency is an enemy when we're so far back."
UPDATE: 13:00 30/09/09
Mr Cameron said he was dlighted by the decision of The Sun to switch its support from Labour to the Conservatives, but maintained that voters would decide the election result.
"In the end elections are decided by people... not by newspapers, but I'm delighted that The Sun, that is a really important national newspaper... has come out and said it's going to support the Conservative party because we are setting the agenda in British poltics,"
"I want to have the widest possible, broadest possible coalition for change so obviously I welcome any newspaper or business or media organisation that comes on and says the Conservatives have got the right ideas."