Ethics adviser Alex Allan quit after Johnson ruled Patel should stay in post.
Rayner said: “The ministerial code, by which government ministers are bound, clearly states that official facilities and resources may not be used for the dissemination of party political material.
“This includes the prime minister’s new media briefing room, which cost the British taxpayer £2.6 million.
“The prime minister has a lot of experience with the ministerial code – his home secretary was found to have breached it after bullying staff, prompting his independent advisor on ethics and ministerial standards to resign.
“The British people would rightly not expect a prime minister who has spent so much first-hand experience of dealing with matters relating to the Code to be so blatant in flouting it during a pre-election period.”
Rayner states that in the four years Sadiq Khan was mayor before Covid hit, he reduced the operating deficit of TfL, left by the previous mayor, by 71%.
But Johnson said during the press briefing on Monday: “As for the finances of TfL I must respectfully remind you that I left them in robust, good order. It is not through any fault of my own the current Labour mayor decided to blow them all on an irresponsible fares policy.
“We are doing our best to help them out and we will continue to do so. But I’m afraid you have to look at some of the decisions that were taken by the current Labour mayor as well.
“I hesitate to make a point like that but since you rightly draw attention to the fact I’m a proud former mayor of London I do think we could look at the way TfL is being run.”
HuffPost UK has contacted the Cabinet Office for comment on the letter.
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