Tony Blair's U-turn on holding a referendum on the EU constitution has prompted the Daily Express newspaper to switch its backing to the Tories.
Desmond gave £100,000 to Labour in adverts before the 2001 polls
The Express backed Labour at the 2001 election but is now describing the vote change as a "final act of betrayal".
The newspaper has been a Tory backer for much of its life but its owner, Richard Desmond, donated £100,000 to Labour before the last election.
Earlier, Mr Blair clashed with Tory leader Michael Howard over the constitution.
The Daily Express: Tory 1992, 1997, Labour 2001
The Guardian: Labour 1992, 1997, 2001
The Mirror: Labour 1992, 1997, 2001
The Daily Telegraph: Tory 1992, 1997, 2001
The Sun: Tory 1992, Labour 1997, 2001
The Independent: Neutral 1992, Labour 1997, 2001
The Times: Tory 1992, Eurosceptic 1997, Labour 2001
The Daily Mail: Tory 1992, 1997, 2001
The Daily Star: Neutral 1992, Labour 1997, 2001
The Express has been vitriolic about the government recently so its switch will come as no surprise.
Its circulation is around 940,000 - relatively low for a tabloid - but its change of allegiance is likely to be seen as a step towards a revival for the Tories, who have also tried to woo press baron Rupert Murdoch.
Mr Desmond is famed for adult magazines like Asian Babes, which he recently sold off. Labour faced criticism for taking money from a "pornographer".
In Thursday's edition, which carries the front page headline "Enough Is Enough, Mr Blair," Express editor Peter Hill says the prime minister has now forfeited the trust of the British people.
"Mr Blair's cynical U-turn on the European constitution, offering a referendum AFTER the next general election - a referendum that will probably never happen - is the final act of betrayal," he said.
He describes the government as "dismal and tawdry" and says Conservative leader Michael Howard deserves to be given a chance.
As well as fears that the new EU constitution will lead to a European super state, the newspaper complains of a "flood" of immigrants, rising taxes and "shambolic" public services.
Mr Hill told BBC Radio 4's Today programme the change of allegiance was "my idea entirely," although he said he had consulted Mr Desmond on the switch.
Home Secretary David Blunkett told BBC Radio 5 Live he was not worried about the shift, as the paper was already pursuing a right wing agenda.
The change follows a prime minister's questions session which saw MPs from both sides heckled and jeered as the two leaders slugged out their opposing positions on the EU constitution.
In fierce exchanges, Mr Blair rejected Mr Howard's challenge to hold a televised debate on the constitution, saying the issue would be discussed in Parliament then in the country.
The prime minister also hinted he could hold a second referendum on the European constitution if the public said "no" first time round.
COUNTRIES HOLDING OR LIKELY TO HOLD A REFERENDUM
Asked what he would do if there was a "no" vote while he was still in office, Mr Blair said: "We will be in exactly the same position as Ireland after its rejection the first time around of the Nice Treaty.
"Which means that if we were in government, we would sit down and have to discuss the way forward with other European countries."
Ireland rejected the Nice Treaty in a referendum only to be ratified by a repeat of the poll 16 months later.
The comments prompted a fresh offensive from the Tories.
In a speech to the British Chambers of Commerce, Mr Howard said the government planned to renegotiate the treaty if there was a "no" vote.
"In other words, if the British people did not vote the way he wanted, Tony Blair would make them vote again until they did," he said.
STEPS TO A REFERENDUM
Constitution expected to be agreed in June
Parliamentary approval may be sought before a vote
It could also take place alongside referendums for English regional assemblies this autumn
A vote could also be held at the time of the general election predicted next spring
It is also possible a vote will take place after the next general election
As the battle lines were set out, the prime minister suggested the Tory stance would transform Britain's relationship with the EU and leave other nations going ahead without it.
In a letter, he accused the Tory leader of dodging questions on whether he would reject a treaty in its entirety or renegotiate if there was a "no" vote.
Mr Howard replied: "If the British people were to vote 'yes', a Conservative government would accept the constitution.
"If the British people were to vote 'no', a Conservative government would veto the constitution: and we would not agree to any new treaty which establishes a constitution for the European Union."