A survey of key marginals for the News of the World suggested that David Cameron's party is still not on track to secure an overall majority.

ICM questioned voters in 96 Labour-held constituencies where the Conservatives need a swing of between 4% and 10% to win. They found that the Tory support had fallen by four points to 36% since January, while the Lib Dems had surged five points to 19%. Labour remained on 37%.

According to ICM, the figures would be likely to leave Mr Cameron in control of 308 seats after the General Election - not enough to wield a majority. Labour would have 248 seats and the Lib Dems 61.

ICM researcher Martin Boon said: "Although the Tories still hold a 6% swing from Labour in these seats, this is smaller than the 8% swing we saw in them back in January and only slightly above the national swing of 5% averaged over ICM's last few polls.

"These results have important implications for seats in the next House of Commons, with a hung Parliament a real prospect."

However, Mr Cameron can draw comfort from the fact that 47% of those questioned believed the Conservatives had won the opening skirmishes of the campaign, compared to 15% who thought Labour had done best. Some 58% also said they had yet to make up their minds who to vote for.

Meanwhile, a national poll by YouGov for the Sunday Times found that the Tories' advantage over Labour dipped from 10% to 8% over the past week.

But Mr Cameron once again achieved the magic 40% support level experts generally believe is needed for outright victory.

The Conservatives were up one point, while Labour was up three at 32%, and the Lib Dems were down two on 18%.