Labour mayoral hopeful Glenda Jackson is set to find out which of her rivals her supporters want her to back if she fails to become her party's candidate.

The former junior transport minister wrote to her 1,500 Labour Party constituency members asking them whom they think she should give her second preference to in the ballot to choose the party's candidate.

The ballot means that Ms Jackson, MP for Hampstead and Highgate, could have a pivotal "kingmaker" role in the contest between herself, Frank Dobson and Ken Livingstone.

It is not clear whether Ms Jackson, who receives the figures today, will make the result public.

Ms Jackson's team sent out the ballot papers last month - but the result may not even be announced until after the party's polls close on 16 February.

Too close to call

Many commentators believe the ballot between the former health secretary, the Labour leadership's favoured candidate, and Mr Livingstone, the left-wing ex-leader of the GLC, is too close to call.

Ms Jackson's campaign suffered a blow when Ealing's Stephen Pound, previously the sole MP to back her mayoral bid, defected to the Dobson camp.

Electoral system

The party is choosing its candidate by using a complex electoral college.

The system comprises 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 weighting means that an MP, MEP or GLA candidate's vote is worth far more than a party member's vote.

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.

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.