Gaining the backing of the giant union will inject momentum into any campaign by the Brent East MP to succeed Sir Sadiq Khan in City Hall.
Ms Butler, who chairs the London group of Labour MPs, told how being Mayor of London would be the “honour of her life”.
However, she denied a report that she was set to launch on Wednesday a bid to be Labour’s mayoral candidate.
“While I have always said that being Mayor of London would be the honour of my life, and I have never hidden my desire to run for Mayor of London one day, it is simply the case that there is no vacancy at present,” she explained.
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Dawn Butler
PA Archive
“If there is a vacancy one day, I will absolutely be putting my name forward,” she added.
“This is the best city in the world. I am a Londoner through and through, and I have big plans and ambitions if one day I am lucky enough to be chosen by Londoners to represent them as mayor.
“In the meantime, I continue my work proudly representing the people of Brent East in Parliament.”
The GMB has two regions which cover London, split by The Thames.
Sir Keir Starmer takes a selfie with Dawn Butler, MP for Brent East and Mayor of London Sadiq Khan at a reception during the Labour Party Conference in Liverpool
PA Wire
The Standard understands that the London region (which includes East Anglia and the East of England) and the Southern region (which covers from the capital down to the South coast) are both backing Ms Butler in her mayoral ambitions.
Speaking to the Standard he said: “I have had three big jobs in my life.
“I was a lawyer for 11 years and I thoroughly enjoyed being a lawyer… There was nothing I would have given that up for but being the MP for Tooting, the area I was born and raised in.
“And I loved being an MP and minister and there was nothing I would have given that up for but being the Mayor of this city that I love.
“Maybe Liverpool will get in touch with me and say do you want to take over the club as a manager?
Mayor of London Sir Sadiq Khan has yet to decide whether to seek a fourth term in City Hall
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“There are one or two dream jobs I would give up this job to do but let’s wait and see if they contact me.”
“When I was growing up I never dreamt I could be the Mayor of this great city,” he said.
“Everyday I have a pinch me moment, I feel like the boy with the golden ticket.”
He stressed that key to the job as mayor was fulfilling the “London promise” in such a diverse city with so many opportunities.
He said: “The London promise is very simple: you work hard, you get a helping hand and you can achieve anything. And my family has benefited from the London promise.
Shadow Communities Secretary Sir James Cleverly who is mulling seeking to be the Tory candidate for Mayor of London
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“I’m proud to have been the mayor that introduced universal free school meals.”
He also highlighted his record on crime, the bus hopper fare, the Elizabeth Line, the Northern Line extension, the Superloop and youth clubs.
Olympic gold medallist Lord Coe’s name has also been mentioned as a possible contender.
Nigel Farage’s Reform UK has chosen Laila Cunningham, who defected from the Tories, as its mayoral candidate.
