Labour's lead over the Conservatives has stretched to 12 points, its largest in more than seven years, according to a ComRes poll.
Ed Miliband's party is on 43 per cent, up 2 points on last month, while David Cameron's Conservatives are on 31 per cent, a fall of two points. The gap is the largest for ComRes since a poll in March 2005, when Tony Blair was about to lead the party into his final general election campaign.
If repeated at a general election, Labour, on current constituency boundaries, would win with a 120-seat majority. Two months ago, Labour's lead was just four points, but Mr Miliband has enjoyed a surge in his party's ratings after his "One Nation" conference speech.
The poll was taken between Wednesday and Friday, before the full results of the by-elections and police and crime commissioner votes on Thursday had become clear. Mr Miliband was jubilant on Friday after Labour's Andy Sawford won the Corby by-election with a swing from the Tories of 12.7 per cent, declaring it a victory for "One Nation Labour" in the heart of Middle England. However, the swing was less than the 17 per cent the Tories achieved in the Crewe and Nantwich by-election in 2008 and the circumstances of the Corby by-election are unusual, with outgoing Tory MP Louise Mensch facing criticism from her constituents for quitting.
Mr Miliband is delivering a speech on Europe tomorrow at a time of maximum difficulty for the Prime Minister on the subject, and his party could once again join Tory rebels in defeating the government on the EU following this week's budget summit.