Sadiq Khan
Labour's candidate for City Hall made the announcement after being attacked by Boris JohnsonGetty

Sadiq Khan has been forced to rule out appointing a close Jeremy Corbyn ally as the chair of Transport for London (TfL) after being challenged over the issue by Boris Johnson. The top Tory claimed Labour's mayoral candidate would ask Ken Livingstone to take on the role if he was elected to lead City Hall in May.

"There is a huge risk that London is about to lurch backwards to the Jurassic age, ruled by saurian socialists such as Livingstone and Corbyn," Johnson, the current Mayor of London, wrote in his Daily Telegraph column.

"Passengers who desperately need continued improvement are in danger of becoming Labour's lab rats – a Corbynista experiment under Khan. Don't let it happen. Back Zac and crack on with modernising the greatest city on earth."

But Khan took to Facebook to rebuke the Uxbridge and South Ruislip MP and accuse Johnson of telling "porkies". He wrote: "Livingstone will not serve as chair of TfL during my mayoralty. It's too important a role to leave to someone else – I'll always do the job myself."

He also claimed there had been more than twice as many Tube strikes since Johnson became Mayor of London in 2008 than in the previous eight years.

The latest attack from the Tories and the party's candidate, Zac Goldsmith, comes after the unveiling of a poster linking Corbyn and Khan. The Conservatives claimed Khan will be Corbyn's man in City Hall instead of London voters, citing the MP's nomination of Corbyn ahead of the Labour leadership race and donations from Tube unions the TSSA and Unite.

The latest YouGov opinion poll, of more than 1,000 people, put Khan 10 points ahead of Goldsmith in the second round of voting (55% versus 45%, respectively).