Support for Labour is plummeting, according to new voter research.

The latest Guardian ICM opinion poll puts the Tories on 40%, the party's highest rating in the newspaper's surveys since March 2010 and five points ahead of the Opposition.

Separate research carried out by Populus for The Times sees Labour on 38 points, down one since December, while the Tories gained two taking them to 37.

It also reported the number of voters who "find it difficult to imagine Ed Miliband running the country as prime minister" was growing - up from 63% in September to 68% in the latest survey.

Read Peter Kellner from YouGov's blog on HuffPost UK about why they're so confident of the Tories' lead in the polls.

Today's polls tally with those taken at the weekend, which either showed the two main parties neck and neck, or the Tories taking a significant lead

The YouGov survey for The Sunday Times put the Conservatives on 41% compared with 36% for Labour - the biggest Conservative lead since autumn 2010. The Liberal Democrats trail on 9%.

Meanwhile a ComRes poll for The Independent on Sunday and Sunday Mirror put the two main parties level-pegging on 38%, with the Tories up two points from last month and Labour down two. The Liberal Democrats are down one point on 11%.

However the poll suggested Labour would do better if Ed Miliband was replaced as leader by his brother David - which would see them gain a three-point advantage - 38% to 35% - over the Conservatives, while the Lib Dems drop to 9%.

With David Miliband as leader, Labour attracts support from 8% of Tory voters and 15% of Liberal Democrats. The polls will be a continuing source of concern for Labour, which has watched the clear poll lead it enjoyed throughout much of 2011 evaporate. Taking in the margin of error, most polls in the past two months have the two big parties neck-and-neck.