Gordon Brown pulled back yesterday from disciplining a Labour MP who has agreed to work for Boris Johnson if he becomes London's mayor after deciding that it would add to Labour's already acute embarrassment.

Kate Hoey escaped punishment despite agreeing that she would work as an adviser on sport and the 2012 Olympics if Mr Johnson won in tomorrow's mayoral vote.

Labour MPs reacted with impotent fury after Mr Johnson's camp put out the news, calling Ms Hoey “the first member of his administration”.

Ms Hoey is unpopular with the Labour whips because of her regular rebellions against the Government. The initial instinct among senior MPs was to urge her expulsion from the party.

But after frosty conversations with Geoff Hoon, the Chief Whip, and Jack Straw, the Justice Secretary, Ms Hoey made plain that she had not been party to Mr Johnson's decision to announce the news and said she would be supporting Labour candidates in tomorrow's elections.

In so doing, she stayed within the rules, even though Labour MPs accused her privately of behaving disloyally. One called her an “utter disgrace”.

But ministers also knew that to have taken any further action against her would have given another boost to Mr Johnson, and decided that doing nothing would be to make the best of a bad job. One said: “She's not worth the fuss.”

Mr Johnson had said he was “delighted” and that it showed he was determined to “bring talent from across politics and the community to a new administration”.

But Ms Hoey insisted that she had agreed only to act as a non-partisan adviser on sport and the 2012 Olympics and in no way backed the opposition candidate in his battle with the incumbent Mayor, Ken Livingstone.

She compared her potential role with that played by Tory MPs Patrick Mercer and John Bercow as advisers to Gordon Brown on terrorism and learning difficulties.

In a statement she said: “The key part of the Boris Johnson statement - ie, that I will be the first member of his administration - is wrong. I have simply agreed to act in a similar position, for example to the Conservative MPs John Bercow and Patrick Mercer in that I have said that I will advise on a non-partisan basis in respect of my lifetime commitment to bringing sport to the people of London.

“This is not an endorsement of Boris Johnson for mayor. I will be voting for my party and Labour candidates on Thursday,” she said, although she failed to mention Mr Livingstone by name.

“I am a Labour MP and I am standing for Labour at the next election. I support the Labour Government. I have and shall continue actively to campaign for Labour in these elections, not least for Val Shawcross, my Greater London Authority member.”

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 rag-bag of people whose views are completely rejected by Londoners”.

Mr Livingstone dismissed his former parliamentary colleague's “eccentric” views. “I'm surprised he's going to take her advice on sport because I think the reason Tony Blair sacked her at the end of his first term was because she'd been involved in all the fiasco over Wembley,” he said.

“But I suppose she knows more about it than Boris does,” he told LBC Radio.

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