The Labour Party's lead is now at 13 points, according to today's Guardian/ICM poll - down by two points on last week's poll.
The Liberal Democrats have picked up votes from Labour since the start of the campaign.
Mr Charles Kennedy's party is up four points to 17 per cent, compared with the 13 per cent share of the vote they enjoyed in March, while the Conservatives have failed to make any headway since the campaign started. The Lib Dems are up one point since last week's poll.
The 13-point lead, according to the ICM "variometer", which takes account of regional variations in swing, would see Mr Tony Blair's second-term majority cut by around 34 seats to 145.
Party political strategists will expect Labour's lead to narrow further as the campaign goes on and will regard the result of this survey as more credible than the leads of 26 and 28 points reported in Mori polls in the last seven days.
The results of the second weekly Guardian/ICM poll of the campaign - taken over Sunday and Monday - also show that Labour has increased its lead over the Conservatives on the single issue of tax to 29 per cent to the Tories' 20 per cent, but that the Tories are now only one point behind Labour when voters were asked which party has the best policy on crime.
While nearly one in three Tory voters wants to see Mr William Hague step down as leader after the election, 62 per cent believe that he should keep his job.
The detailed results show that Conservative voters appear to have been increasingly impressed by Mr Hague's performance since the election started.
A month ago only 49 per cent of Tory voters nominated Mr Hague when asked who should be prime minister. Today's poll shows that Mr Hague's campaign performance has now pushed that figure up to 61 per cent of the Tory faithful.