Transport for London's commissioner downplayed risks of delay on the £17.6 billion Crossrail scheme and should consider resigning, a report has said.
Mike Brown, who has held the role at TfL since September 2015, is said to have "altered key messages of risk" on deadlines on the project which were sent to the London mayor's office.
Sadiq Khan's office said he had every confidence in Mr Brown and said the previous leadership of Crossrail were responsible for providing "inadequate information" about the delays.
Crossrail, which will be known as the Elizabeth Line when it opens, was due to launch in December but now could be as late as 2021.
An inquiry by the London Assembly said senior staff at Crossrail were unable to "push past their obsession" with a December launch for the east-west railway, despite an independent review in January 2018 suggesting there were significant risks to the proposed opening date.
"Although in theory (services firm) Jacobs was contracted to help the Sponsors scrutinise Crossrail, in practice their advice was largely missed or ignored," the report, called Derailed: Getting Crossrail back on track, said. It added:
The report also looked at the weekly briefings given to Mr Khan's office which were written and reviewed by Crossrail and TfL staff and signed off by Mr Brown, saying: "There have been several instances where the Commissioner has altered key messages of risk."
In one example, references to problems being caused by signals and train software were removed, with a senior TfL official saying it "was amended by Mike so that the setbacks appeared less serious".
It recommended Mr Brown, appointed by Boris Johnson when he was mayor and paid at least £350,000 in 2017/18, "reflect(s) on whether he is fit to continue to fulfil his role".
Chairwoman of London Assembly's Transport Committee, the Liberal Democrat Caroline Pidgeon, said Crossrail had found itself in "a mess of overspending, mismanagement and an embarrassingly long delay".
Sir Terry Morgan resigned as chairman of HS2 Ltd and Crossrail Ltd - a TfL subsidiary - in December, but Ms Pidgeon said Mr Brown was "at the centre of decisions to dilute important information sent to the Mayor".
"Crossrail will provide immeasurable benefits to London once launched, but vital lessons must be learned by the Mayor, TfL and Crossrail so we all can bring this sorry chapter of the project's journey to a close," she said.
A TfL spokesman said: "It is clear that the responsibility for the delay to the Crossrail project lies with the former management of Crossrail Ltd.
"It is entirely incorrect to suggest the Transport Commissioner, or anyone at TfL, kept any information from the Mayor."