Labour enters the general election to be announced by Tony Blair today with a narrow and shrinking lead over the Conservatives, according to an NOP poll for The Independent.

The survey shows that Labour's lead over the Tories has dropped from five percentage points three weeks ago to just three, its lowest since last October. In February, Labour enjoyed a 12-point advantage, but the momentum since then has been with the Tories, who are within striking distance as the campaign begins.

The new poll, taken between Friday and Sunday, shows that Labour is on 36 per cent (down three points since NOP's last survey), the Conservatives are on 33 per cent (down one) and the Liberal Democrats on 21 per cent (up two). The rise in support for Charles Kennedy's party suggests that it has halted the slide it suffered when it was squeezed during the heated battles between the two main parties during the pre-election campaign.

Although the figures, if repeated at the election on 5 May, would still give Mr Blair an overall Commons majority of around 80, the sharp downward trend since February will worry Labour strategists. Last night, they put a brave face on NOP's findings, saying that a closely fought contest would make it easier to persuade disaffected Labour supporters to vote in an election in which turnout could prove crucial.

Labour's problems in getting its traditional backers to turn out are highlighted by the NOP poll, which found that only 64 per cent of Labour supporters say they are certain to vote compared with 77 per cent of Conservative supporters.

Today, Labour will warn people who opposed Mr Blair on Iraq that they could let Michael Howard into Downing Street "by the back door" if they abstain or switch to the Liberal Democrats.

The NOP survey found that, by a majority of 60 per cent to 40 per cent, people believe Mr Howard was wrong to sack Howard Flight as an MP after he suggested that the Tories would cut public spending by more than the £35bn they admit. However, the Tory leader's punishment is backed by Tory supporters by a margin of 59 per cent to 41 per cent.

NOP found that the Tories had not yet convinced people about their strategy of cutting taxes while protecting key public services such as health and education. By a margin of 52 per cent to 41 per cent, people do not believe it is possible to do both.

Despite the controversy caused by Mr Flight's remarks, the Tories have made some headway on tax and spending since last November, when 37 per cent thought it was possible both to cut taxes and preserve services and 58 per cent did not.

Two other polls published today also show that the gap between the two main parties is closing. A Populus survey for The Times gives Labour a two-point lead, while ICM for The Guardian shows Labour with a three-point advantage.

Mr Blair will go to Buckingham Palace today to ask the Queen to dissolve Parliament next Monday, allowing the election to be held on 5 May.

Launching the campaign, Mr Blair will highlight the election as a choice between two different visions for Britain's future offered by Labour and the Tories. He will say that the election should be about that choice and not be seen as a referendum on Labour's record since 1997.

Gordon Brown, the Chancellor, will say in a speech: "The election is not just about who is to govern, but the future direction of the country. In the campaign ahead, Britain will face a fundamental choice that will not only determine the future of the economy but the strength of our society." He will argue that Labour, by delivering eight years of economic stability, has earned the right for a new mandate to tackle the new challenges facing Britain from China and Asia.

The election campaign was originally to be launched yesterday, but was postponed for a day after the death of the Pope. Mr Blair may have to scale down his plans.

After the leaders do battle at the final session of Prime Minister's Questions before the election tomorrow, there will be a pause in electioneering for the Pope's funeral on Friday and the royal wedding on Saturday. The campaign will start next Monday.

Meanwhile, Conservatives in the former constituency of Mr Flight drew up a shortlist last night of three possible replacements as a new candidate for the election. Anne Marie Morris, Laura Sandys and Nick Herbert, director of the think-tank Reform, were selected at an executive committee meeting at the Arundel and South Downs Association. Mr Flight's successor will be determined at a full constituency meeting tomorrow.

The announcement came after Mr Howard rejected a last ditch appeal from Mr Flight to allow him to stand in the pending elections.