The Tories are still trailing Labour in marginal seats they need to win if they are to secure a Commons majority, according to a poll published today.
The Ipsos Mori survey for Thomson Reuters provided further evidence that a hung parliament was on the cards at the forthcoming general election.
It found that in constituencies set to determine the outcome of the election, Labour leads the Tories by four points among those certain to vote.
The poll was taken in 56 Labour-held seats that the Tories need a swing of between 5% and 9% to win.
David Cameron needs a national swing of about 7% towards the Tories to get a majority of MPs and form the next government.
In the marginal seats covered by the poll, Tory support has risen since the 2005 General Election but not enough.
Then, Labour polled 45% compared with the Tories' 31%. This week Labour was on 41% and the Tories 37%.
The poll was taken before yesterday's Budget, the effects of which on the state of the parties is not yet clear.
But the findings reinforce a slew of recent national polls suggesting that the Tories are short of the support they need to secure a majority in the Commons.
Helen Coombs, deputy head of political research at Ipsos Mori, said: "This is very much hung parliament territory.
"Nevertheless, everything is still to play for, since almost half the public, and a third of those who are certain they will vote, say they may still change their mind."
* Ipsos Mori interviewed 1,007 adults across 56 marginal constituencies which are Labour-held and the Tories need a swing of between 5% and 9% to win. Fieldwork took place between March 19 and 22.