Gordon Brown last night dismissed as "Tory spin" suggestions that a senior Labour MP had broken ranks to join Boris Johnson's team if he were to win the London mayoral contest on Friday.

Just two days before the polls in England and Wales, Kate Hoey, who represents Vauxhall in central London, was said yesterday morning to have joined Mr Johnson's team as an unpaid non-executive director, having become "the first member of his administration".

The Tory candidate was "delighted" at the political coup while David Cameron, the Conservative leader, said she would be a "good addition" to Team Johnson, adding: "She has always been a great Member of Parliament." On the Labour benches, however, there was anger and dismay.

Ken Livingstone, looking for his third term as mayor, dismissed Ms Hoey as "eccentric" and claimed she had always been "a sort of semi-detached member of the Labour Party".

At Westminster, one back bencher branded her action "deplorable", coming just ahead of Thursday's polls, while Lord Moonie, the former Defence Minister, noted: "She has overstepped the mark".

There were suggestions some Labour colleagues had called for her to be kicked out of the party - endorsing a candidate from an opposing party is a sackable offence - and that, after a meeting with Geoff Hoon, the Chief Whip, she rushed out a statement of clarification.

The former Sports Minister insisted she had agreed to act only as a non-partisan adviser on sport and the 2012 Olympics and in no way backed Mr Johnson in his battle with Mr Livingstone.

"The key part of the Boris Johnson statement - ie, that I will be the first member of his administration - is wrong," declared Ms Hoey.

She explained how she had agreed to act as an adviser, similar to those Tory MPs who were advising Mr Brown in his "government of all the talents".

"This is not an endorsement of Boris Johnson for mayor. I will be voting for my party and Labour candidates on Thursday." However, it was noticeable that she failed to mention Mr Livingstone by name.

She has had a few run-ins with her Labour colleague. In January, Ms Hoey complained about a lack of scrutiny of his City Hall advisers and a "cult of silence" which stopped people speaking out about alleged malpractice. Last week, the mayor was said to be "furious" after she agreed to attend an event with Mr Johnson but withdrew at short notice.

Last night, a spokeswoman for the Livingstone campaign accused Ms Hoey of having backed Paris over London for the 2012 Games and said Mr Johnson was "assembling a ragbag of people whose views are completely rejected by Londoners".

Earlier on the stump, Mr Brown appeared wrongfooted when he was faced with a question about Ms Hoey during a visit to Bletchley near Milton Keynes.

He initially turned away from the reporter to answer a different question and then returned to it a minute or two later, insisting: "She has answered the question herself and this is an example of Tory spin."

The Hoey row broke out as the UK Government announced it would hold the Westminster by-election in Crewe and Nantwich, following the death of Labour veteran MP Gwyneth Dunwoody, on May 22.

With a majority of just in excess of 7000, Tory strategists will be hoping the party can gain its first parliamentary seat in a by-election since Mitcham and Morden in London in 1982.

Yesterday, No 10 confirmed the weekly cabinet meeting was dominated by discussion of the UK Government's legislative plans and revealed that Mr Brown will again publish a draft Queen's Speech.

However, this could come as early as next week and form the centrepiece of the PM's fightback following what is expected to be a raft of bad council results in England and Wales and, possibly, the loss of the London mayoralty.