When the Sun famously changed tack and swung behind Labour during the British election campaign last year, the widely held belief was that the newspaper secured Mr Blair's victory. It seems however, that the love affair with Labour is over.
"Is this the most dangerous man in Britain?" the Sun asked yesterday, alongside a photograph of the Prime Minister. The problem? Mr Blair's determination to scrap the pound and take Britain into the European Single Currency.
It was a question they said they never dreamed they would ask, but the Sun accused Mr Blair of taking the "biggest gamble" ever if he goes ahead with the single currency. It had backed Mr Blair during the election because he was the best man for the job, but the euro was a different issue.
In its editorial, the Sun accused Mr Blair of using his popularity "to cajole, seduce and persuade the voters to back him" over the single currency. And the Sun warned: "We are against it economically, politically and constitutionally. We will fight, fight, fight."
Challenged in the Commons over the Sun's headline, Mr Blair insisted the government would remain committed to preparing to join the single currency if it was in the national interest. In a stinging response to the Sun, he said: "Newspapers are entitled to their view, but we govern in the national interest."