Despite a surprise tax cut and frantic campaigning by senior Government ministers, the Tories have doubled their lead in Crewe and Nantwich over the past week.

And there was no comfort for the Prime Minister at a national level, with a separate survey giving David Cameron a massive 20-point advantage.

Research by ICM for the News of the World found that 45% of Crewe and Nantwich constituents are planning to vote Conservative, compared with 37% backing Labour.

If repeated on Thursday, the result would destroy Labour's 7,000 majority and deliver Mr Cameron a new MP by a margin of more than 1,000.

The survey suggested that this week's "mini-Budget", which handed a £2.7 billion windfall to basic rate taxpayers, had backfired.

Nearly half of voters said the package - designed to defuse anger over the abolition of the 10p band - was the right thing to do.

But 59% also viewed the move as a cynical pre-election bribe, and 24% claimed it would make them less likely to support Labour.

Just under two-thirds were convinced that the Government would have a better chance of winning a General Election without Mr Brown in charge.

Meanwhile, a YouGov poll for the Sunday Times put the Tories on 45% to Labour's 25%, with the Lib Dems on 18% - up two points since last month.