If it's The Sun wot wins it, Tony Blair has already secured the keys to Downing Street for a second term.

The front page of the tabloid newspaper which famously declared it had helped the Tories to victory in 1992, announced on Thursday it would back Labour at the forthcoming election. "It's in the bag, Tony," trumpeted the headline, as it applauded Gordon Brown's Budget.

The paper, which accused William Hague of "desperation" in his Harrogate speech on asylum seekers last weekend, made dismal reading for the Tories, who now face a dwindling band of supporters in Fleet Street.

In truth, The Sun's backing for Labour was not entirely unexpected, particularly as Trevor Kavanagh, the paper's veteran political editor, had dropped several hints in recent weeks.

Michael Portillo, shadow chancellor, attempted to put a brave face on the announcement yesterday: "I was not actually expecting The Sun to come out in favour of the Conservative party. We have seen the signs for a long time."

What he did not say - but many Tories must secretly acknowledge - is that The Sun would not want to risk backing the losing horse.

The two papers that can be virtually guaranteed to back the Tories are The Daily Mail and The Daily Telegraph. But, to judge by its current editorial stance, The Times, which in 1997 advised readers to back candidates on their opposition to further European integration, would not be expected to come out in favour of Mr Hague. The Mirror and The Guardian are left-wing, and The Independent - while loath to pledge allegiance to a party, would not throw its weight behind the Tories. Even The Express, which flirted with Mr Blair last time but ended up sticking with Mr Hague, is expected under its new proprietor to endorse Labour.

The Financial Times, which has backed Labour two elections running, has yet to come to a decision. Its editor, Richard Lambert, said: "Unlike the Sun, the FT is a family newspaper and will make up its mind not just on the Budget, but on broad issues."

Labour officials were understandably pleased. But even they questioned how influential newspapers were in persuading people how to vote. One said: "We do not take the next election for granted and we will be working to get the message across . . . regardless of the outlet."

In addition, The Sun's allegiance cannot be relied upon. The government has been keen to tread carefully to keep Rupert Murdoch, whose News Corporation is the ultimate owner of both The Sun and The Times, on board. But business decisions taken in the next parliament may not guarantee his continued support.

Even on Thursday, Mr Kavanagh, who Tory officials believe is sympathetic, said Mr Brown should have taken a leaf out of US president George W. Bush's book and showered the electorate with tax cuts. Another Labour official said The Sun would have a decisive impact only in a "tight fight".

All the opinion polls suggest this election is likely to see another big Labour win, so the chances are most voters will have made up their minds before reading the tabloid newspapers.