Yesterday Chris Grayling rejected Sadiq Khan's bid to take over commuter routes, saying the mayor hadn't proved his alternative would improve services.
But in the leaked letter seen by the Evening Standard, Grayling had written to former mayor Boris Johnson before Khan took over, saying he was against rail devolution to stop services being under the control of a Labour mayor. At the time – in April 2013 – Grayling was justice secretary as well as MP for Epsom and Ewell.
Deputy Leader of the Labour party Tom Watson called the letter "a disgraceful revelation".
"Labour’s mayor of London put forward a plan which would see commuters enjoy a better service and frozen fares," he said. "We now see the Tories have blocked this progress for their own narrow political interest."
Watson added: "It’s no wonder even Tory MPs are today calling for Chris Grayling to resign: he’s in serious hot water."
Conservative MP Bob Neill led the calls, saying he thought Grayling had compromised his position "and he should resign" after acting for party reasons.
Addressing the problems rail users encounter on @Se_Railway must be the priority, not party politics. I won't support DfT's cynical decision— Bob Neill (@neill_bob) December 7, 2016
Labour London Assembly member Andrew Dismore criticised the note, warning it looked like "political point scoring".
"It's the greatest shame for passengers that the minister's political point scoring has seemingly taken precedence over their needs," Dismore said.
The fact remains that where Transport for London (TfL) has managed services, we've seen some of the best performances. If allowing TfL to manage suburban rail franchises will mean paying passengers get better, more reliable services, then we need to move pass this pettiness and make it happen.
And the mayor said "a good rail service for commuters is far, far more important than party politics" in response to the leak.
The Department for Transport has been approached for comment.
Rail devolution secured cross-party support from MPs and the London Assembly. Earlier in the year, Johnson and Grayling's predecessor Patrick McLoughlin said TfL would take control of suburban rail as each franchise cropped up for renewal.
But in his statement to Parliament on plans for revamping Britain's rail network, Grayling said he will be inviting TfL to be more closely involved in developing the next Southeastern franchise, through "seconding a TfL representative".