The Tories have opened up a five-point lead over Labour, according to a new opinion poll.

The YouGov survey for The Sunday Times puts 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 puts 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 suggests 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.

Andrew Hawkins, chairman of ComRes, says: "Chuka Umunna and Yvette Cooper both are too low profile to reflect fairly how they would rate against the Conservatives." Their don't knows (excluded from the figures above) are 50 and 45% respectively.

According to ComRes, the poll also suggests that Ed Balls's change of emphasis this week has gone down badly with Labour voters. Only 43% (and only 1 in 3 Labour voters) agreed with Balls's claim last week that a Labour government should stick with the government's cuts agenda.

:: YouGov interviewed 1,711 adults across Britain online on January 19 and 20. ComRes interviewed 2,050 GB adults online on January 18 and 19.