Labour's lead in the daily You Gov tracker poll grew to the highest size ever recorded by the firm today.
The opposition is now enjoying a 15-point lead, with Labour on 45% and the Conservatives on 30%.
The extent of the lead confirms suspicions that David Cameron's EU referendum speech had a limited effect on voters' intentions.
Labour's lead had recovered to 12% in YouGov surveys by last Friday and it now appears to be soaring to unprecedented levels.
The poll comes as Chris Huhne's resignation triggered a by-election in Eastleigh.
The seat is a straight Tory - Liberal Democrat fight but Ed Miliband will want to improve Labour's showing to demonstrate a sense of momentum around the party.