Twenty contenders who have applied to be the Tory candidate for London mayor will know today whether Boris Johnson has entered the race at the eleventh hour – probably to dash their hopes.

David Cameron, who was hit yesterday by a poll giving Labour its biggest lead since he took over, would probably welcome the intervention of his colourful, albeit gaffe-prone, higher education spokesman.

In a boost to Gordon Brown, the ICM survey for The Sunday Telegraph put Labour on 40 per cent compared with 33 per cent for the Tories and 19 per cent for the Liberal Democrats.

The seven-point lead – Labour’s biggest since September 2005 – came as it emerged that Ed Miliband, a close Cabinet ally of Mr Brown, had been set to work on Labour’s general election manifesto.

A Johnson candidacy would help the Tories to challenge Labour’s current domination of the airwaves, although some Conservatives believe that it would be a mixed blessing if he slipped up badly. But Mr Johnson again kept everyone, including his party, guessing yesterday as he struggled to make a decision. All the signs are that he will run, but because he is believed to have changed his mind three times already, no one is taking anything for granted.

According to insiders Mr Johnson wants to stand, the party hierarchy is keen for him to do so and his Henley constituency party has come to terms with the idea, but he was still trying to convince his family that it was the right thing to do.

The waiting is almost over. He has until noon, when the Tory nominations close for the right to take part in the London “primary” election, to make up his mind. If he stands, his high profile in a contest in which all Londoners can vote will give him a big chance of being the Tory contender to take on Ken Livingstone, who is running for his third term.

The Conservatives have struggled to find a heavyweight challenger to Mr Livingstone. Previous attempts to woo Greg Dyke, the former BBC Director-General, and Sir John Major, the former Prime Minister, failed.

If Mr Johnson decides to stand he will join the other people who have applied. A committee of senior London Conservatives will whittle them down to a shortlist of five. That process will be completed by the first week of August when those chosen will embark on a series of hustings across the capital. The result will be known in time for the party’s autumn conference.

Yesterday’s poll added to the view among politicians that Mr Brown may go for an early election next year.

In his first speech as Prime Minister to Labour’s National Policy Forum, Mr Brown made an audacious grab for Tory support over the weekend when he invited “moderate Conservatives” to join Labour. He insisted that Labour had to have the confidence, “to shape and enlarge the centre ground” around the party’s values.

In a pointed appeal to Tories, he went on: “I urge all who share our values – from progressives in all areas of Britain to moderate Conservatives – to join us in making Britain the country it can be.”

Mr Brown also promised a series of initiatives on housing, welfare and younger people in the weeks ahead.

Peter Hain, the Work and Pensions Secretary, is to announce an extension of the New Deal that would match up the hardest-to-reach unemployed with job vacancies.

The Prime Minister said that he had signed up companies across the retail, financial services, construction and hospitality sectors that were committed to finding work for inactive young people, single parents and the disabled. There are currently 600,000 vacancies.

“We have signed up large numbers of employers who now tell us they will hire those people who we want most of all to find jobs for,” Mr Brown said.

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