The first survey to compare the two men among the 25 per cent of voters whose decisions tend to determine the general election outcome will be a boost for the Chancellor. After a string of polls suggesting that the Tory leader is ahead generally among voters, the survey gives Mr Brown a 27-point lead among swing voters and is likely to help calm fears among Labour MPs that he might be an electoral liability.

The poll, conducted by The Times pollster Populus for a new think-tank, Opinion Leader Forum, involved interviews with 1,018 people, 242 of them swing voters, from October 13 to 15. It is statistically a small sample; the findings therefore have to allow for a margin of error of at least plus or minus 3 per cent.

The poll defined swing voters as people who are inclined to vote Labour at present, but say there is a fair chance of them going to another party — and those currently inclined to vote for another party or unsure about which one to vote for, but who say there is a fair chance of switching to Labour.

The apparent differences between the electorate at large and swing voters when they weigh up Mr Brown and Mr Cameron and their leadership attributes will give added spice to the political debate as Tony Blair prepares for his departure, with Mr Brown the strong favourite to replace him.

Asked whom they would prefer to be the next prime minister, putting aside party preferences, 51 per cent opted for Mr Brown, and 24 per cent for Mr Cameron, with 17 per cent saying they favoured them both equally. More see Mr Brown as “strong” and as having more “substance”. The interviewees put the pair neck-and-neck on whether they understand ordinary life and have integrity.

Overall, the figures suggest that if Mr Brown was leader, Labour would hold on to most of its current votes and attract some from people currently supporting other parties.

Today’s poll comes as Mr Cameron makes a direct pitch for the “grey vote”, elderly voters who he believes Labour has discriminated against in campaigns such as “Cool Britannia”. He says a cultural change is needed “in the way we think about older people and a revolution in social responsibility in how we behave towards them.”

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