Labour left-winger Ken Livingstone's London mayoral bandwagon has received a major boost after fellow candidate Glenda Jackson said she would use her second preference vote to back him.

Ms Jackson, a former junior transport minister, made the decision after holding a ballot of her constituency party in Hampstead and Highgate.

The decision flies in the face of efforts by the party leadership to persuade ordinary members to back Frank Dobson, the prime minister's choice for the job.

Party members in Ms Jackson's constituency rejected Frank Dobson and voted 147 to 62 in favour of Mr Livingstone.

As a London MP, Ms Jackson is entitled to vote in the MPs' section in the complex electoral college set up by Labour to choose its candidate for mayor.

Ms Jackson told the BBC: "My feeling was that because my vote within the electoral college as an MP is the equivalent of almost a thousand, I was duty bound to consult my constituency party members on how they would wish to see me register my second vote.

"If it had been the other way round I would have abided by that. It seemed to me eminently fair and I think it's something the party can take pride in."

Mr Livingstone welcomed Ms Jackson's announcement, saying: "Glenda's vote alone could decide the outcome of the selection, so it is a good boost at this stage of the campaign."

Ms Jackson is trailing behind the other two candidates and her campaign suffered a blow when Ealing's Stephen Pound, previously the sole MP to back her mayoral bid, defected to the Dobson camp.

A spokesman for Mr Dobson's office said: "We have known for weeks that Glenda Jackson and Ken Livingstone have been working together, this latest episode in their collaboration comes as no surprise.

"We remain confident that the Labour Party members who have supported Glenda Jackson will, and in many cases already have, cast their second preference in favour of Frank."

Electoral system

Last month, Mr Livingstone offered to make Ms Jackson his deputy should he be elected mayor.

At the time, sources close to Ms Jackson told BBC News Online that she was "seriously considering" the offer.

Labour's electoral college is made up of one third London MPs, MEPs and Greater London Assembly (GLA) candidates, one third affiliated trade unions, and a final third of party members.

The system's weighting means that an MP, MEP or GLA candidate's vote is worth as many as a 1,000 votes of ordinary members.

Most MPs, MEPs and GLA hopefuls are expected to follow the party leadership and vote for Mr Dobson.

Unions holding internal ballots are expected to back Mr Livingstone; those not balloting, Mr Dobson.

Poll boost

Mr Livingstone is expected to win a majority in the membership section, but needs around 65% to be confident of victory throughout the whole of the college.

Labour will announce the result of the contest on 16 February.

In a further boost to Mr Livingstone, a London Chamber of Commerce and Industry poll of City business executives suggested he was well ahead of his Labour rivals and only five points behind Tory candidate Steve Norris.

Asked who would be the best mayor for London "from a business point of view", 39% of those who expressed a preference opted for Mr Norris, 34% for Mr Livingstone, 12% for Mr Dobson, 9% for Ms Jackson and 6% for Liberal Democrat Susan Kramer.

However, of the 414 business people quizzed for the survey, some 33% said they did not know which mayor would be best for business.