Brown kept open the option of an autumn election until the last minute by announcing that he will make a Commons statement on Monday on plans to bring home some troops from Iraq. The following day the chancellor, Alistair Darling, will unveil a three-year spending plan and his pre-budget report.

The two-day blitz could provide a launch-pad for an election, which would formally be called on Tuesday after the chancellor's statement. But it could equally be an insurance policy designed to put Labour back on the front foot if Brown decides against an election, which would provoke Tory claims that he had "bottled it".

Initial results of Labour's private surveys in key marginal seats by Stan Greenberg, Bill Clinton's former pollster, are said to be "too close for comfort". But Brown will study the results of more detailed research in the marginals before making the biggest decision of his political career. "The final picture in the marginals will decide it, " one ally said.

A Labour insider said: "Gordon wants to go for it.He has taken every step possible to allow it to happen. But the picture is clouded and a lot of serious people are now telling him not to risk it."

Brown is receiving conflicting advice. Labour MPs in marginal seats are jittery after three opinion polls showed the Tories closed the gap during their conference and one put the two main parties neck and neck.

Labour MP Des Turner told BBC2's The Daily Politics: "Gordon has a perfectly good mandate. He shouldf go to the electorate at the proper time, which I don't think is now." Others say Brown will be damaged if he pulls back. One cabinet source said: "It's gone so far now, I don't think it can be stopped. I think Gordon is going to go for it."

Government whips have been telephoning Labour MPs to find out how the Tory proposal to lift the threshold for inheritance tax to �1m has played in their constituencies. "People are giving the Tories the benefit of the doubt and it's hurting, " a Labour source admitted. Tory calls for an annual limit on immigration are also popular with voters.

In another sign that the government is clearing the decks for an election, it gave the go-ahead on Friday to the long-delayed Crossrail project to improve London's transport system.

Darling warned that the treasury's growth forecast for next year will have to be downgraded because of the global credit crunch. The prime minister's official spokesman said the financial climate would be "testing" but rejected suggestions it would undermine the case for spending increases for health and education to be announced on Tuesday. The Tories stepped up their pressure for an election when David Cameron wrote to Sir Gus O' Donnell, the cabinet secretary, asking him to begin the talks between the opposition and the civil service that normally happen before an election, and sending him his party's plans for government.

Menzies Campbell, the Liberal Democrat leader, criticised Brown for allowing the election speculation. "What we have seen in the last three or four weeks has been what began as a tease, has become something rather more serious . . . really an abuse of the political process, " he said.