The rollercoaster contest to win Labour's London mayoral nomination entered its final stretch on Wednesday as ballot papers were sent out to 48,000 party members in the capital.

And Prime Minister Tony Blair issued a last-ditch appeal to Labour's rank and file not to support grassroots favourite Ken Livingstone MP over his preferred choice, former health secretary Frank Dobson.

All three Labour hopefuls - Mr Livingstone, Mr Dobson and ex-transport minister Glenda Jackson - are expected make a final push for votes as Labour members receive their voting papers.

Members have until 16 February to cast their votes but most are expected to do so by the end of this week.

With the likely final outcome too close to call, Mr Blair and Deputy Prime Minister John Prescott were due to launch a fresh attack on Mr Livingstone at a question-and-answer session for party members in Southall, west London.

Mr Prescott was expected to focus his assault on Mr Livingstone's plans to use bonds to raise money to modernise the Tube, saying they would cost £4.5bn - "That's £4.5bn less to spend on hospitals, schools or tackling crime."

The Labour leader and deputy leader will be hoping to fare better than at a similar meeting last week when Mr Blair and Chancellor Gordon Brown faced jeers and heckles from the audience when the two strongly criticised Mr Livingstone.

Alastair Campbell joins the fray

In a move set to spark controversy the prime minister's official spokesman, Alastair Campbell, also joined the fray earlier on Wednesday.

He said it was a matter for Labour members how they cast their votes in the mayoral contest, but that "if they have any sense" they would take note of the opinions of senior figures such as Mr Blair and Mr Brown rather than of newspapers.

"Providing Labour members focus on the issues, there is no doubt at all who the best candidate is," Mr Campbell said.

Last month Mr Livingstone complained that Mr Campbell, who is technically a civil servant, had abused his public position by crossed the boundary into party political work by interfering in Labour's mayoral contest when he criticised the MP's plans for Tube funding.

Dogged by controversy

The Labour leadership's latest bid to persuade members to support Mr Dobson came the day after party managers bowed to demands for a review of the way future internal elections are held.

The party's mayoral selection contest has been dogged by accusations of dirty tricks and bias on the part of Labour officials. Mr Dobson is facing possible action by the Data Protection Registrar following his use of central membership lists received from the party's Millbank headquarters.

In his latest mailshot to Labour members, sent out this week, the ex-minister said his ambition for the mayorship was to "unite London."

But he renewed his attack on Mr Livingstone: "I have known Ken a long time and he is a very engaging politician."

"However, the truth is that those across the party who know him best don't trust him. Be in no doubt that Ken, who has used this campaign to attack our government, would do so even more vigorously as mayor."

'No negative campaigning' - Livingstone

Each candidate sent out a short election message with the ballots. Mr Dobson's promised full employment and a "relentless drive against poverty".

Ms Jackson said her priority would be tackling homelessness and promised "Midnight Express" Tube and rail services which will run until the early hours of the morning.

Mr Livingstone promised "no negative campaigning by my supporters in this selection" and said he would reach out to Labour's "core support".

Polls consistently showed he had the best chance of securing a large majority which will be "a huge boost to our objective of winning a second term for Tony Blair's government".

Two-horse race

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

But the three-way battle for Labour's nomination has largely been a two-horse race between former Greater London Council leader Mr Livingstone and Holborn & St Pancras MP Mr Dobson.

Ms Jackson, though expected to come in some way behind her two rivals in the final reckoning, has been polling ahead of Mr Dobson.

But the winner of Labour's nomination is being decided by an electoral college 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.

Most MPs, MEPs and GLA hopefuls are expected to follow their party leader 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 through the college.

London's mayoral election takes place on 4 May.