The Conservatives' lead over Labour has been reduced to single figures, according to a Guardian/ICM poll which increases the pressure on Gordon Brown to call an early general election.

While the nine-point lead would probably still give David Cameron a narrow Commons majority, it will reinforce the view in the camps of both party leaders that the Tories can still be deprived of victory. This is the first Guardian/ICM poll – and the first by ICM for any newspaper – since December 2008 to give the Tories less than a double-digit lead.

After 12 months of unbroken Conservative dominance in the polls, the figures – showing the Tories on 40%, down two, Labour on 31%, up two, and the Liberal Democrats on 18% – are likely to intensify calls for Brown to go to the country on 25 March next year, rather than the 6 May polling day that most at Westminster have been expecting.

Labour's vote share has risen for the fourth consecutive month, improving significantly on the 25% share it had in August. The gap between the parties is a sharp reduction from the 17 points recorded two months ago.

The results suggest increasing doubts about the economic competence of Cameron and his shadow chancellor, George Osborne. But there will be some relief at Tory HQ that the party has not fallen below the crucial 40% threshold that gives it a strong chance of winning a majority.

If replicated at a general election, the figures would probably result in an outright Tory win – but by the narrowest of margins. One projection suggests a Cameron government would be returned with an overall majority of four.

The great hope within Labour circles is that the tightening of the race is a trend that may continue to the point where Cameron is denied any kind of victory.

The government's progress in the poll comes at the end of a fortnight in which many MPs believe that, after months of political drift, the prime minister has finally got himself back on the front foot despite a mixed reception for last week's pre-budget report.

His suggestion 13 days ago that Conservative tax policy had been dreamed up "on the playing fields of Eton" brought accusations of class war, but went down well with Labour backbenchers.

The Copenhagen conference on climate change has provided Brown with the chance to present himself as an international statesman, while the new tax on bankers' bonuses – an overwhelmingly popular move backed by 71% of voters, according to today's poll – has also won plaudits from leaders overseas.

The prime minister, however, remains deeply unpopular, and may find it difficult to maintain momentum with no spare public money to play with.

The poll was carried out at the weekend, after news of Alistair Darling's pre-budget report had sunk in. The chancellor's statement confirmed a record-breaking £178bn deficit for this year, and pencilled in spending cuts that will hit the pay of public-sector workers as well as national insurance increases that will hit everyone earning more than £20,000.

The poll shows that 19% of voters believe the package will make the economy worse overall, against 12% who think it will make things better. A mere 31% believe Brown and Darling are better placed to manage the economy than Cameron and Osborne.

While Labour's stewardship of the economy remains unpopular, the narrowing of the political gap appears to be underpinned by growing unease about the prospect of Conservative financial management. Just two months ago, 49% of voters said they thought Cameron and Osborne would do better than Darling and Brown, but that figure is 38% today.

With the small print of the poll suggesting continuing public reluctance to accept the tough choices that balancing the government's books will require, it could be that the Tory tactic of demanding deeper, faster cuts than Labour is willing to make is starting to make the public nervous.

There was speculation about an early election in the weekend press, with Labour chiefs ordering staff to be ready to fight an election any time next year. Brown does not have to go to the country until June. Ed Balls, the schools secretary, said he had "heard nobody talking about going for early or snap elections". But some Labour MPs believe Brown should move sooner rather than later.

Cameron himself sought to encourage the talk – stating in public that he believed that 25 March was a "likely" election date.

Although most recent opinion polls have suggested some tightening in the political race, not all of them have done so. A ComRes survey for the Independent on Sunday suggested that the Conservative lead had actually widened to 17 points.

• ICM Research interviewed a random sample of 1,009 adults aged 18+ by telephone on 11-13 December 2009. Interviews were conducted across the country and the results have been weighted to the profile of all adults. ICM is a member of the British Polling Council and abides by its rules.