Crossrail’s chief executive is to step down, it has been announced, a week after the government agreed to a new cash injection to complete the delayed £15.4bn project.

Simon Wright will be replaced by Mark Wild, the managing director of London Underground, who will remain in charge until the opening of the central section of Crossrail, which will be known as the Elizabeth line.

The line was due to be completed in December, but Crossrail announced in August that it would take up to another year to get trains running on the underground core section. Another £650m of funding was added in June, tarnishing the Crossrail construction team’s claims that the project is meeting its targets.

Last week, a £350m loan was extended to Transport for London, the London mayor’s transport authority, to cover a revenue shortfall caused by the delayed opening. TfL said full-time testing of the route would start soon.

Mike Brown, the TfL commissioner, said Wild had extensive experience of major projects, which would be vital for completing Crossrail safely and reliably. Wild has been a non-executive director of the Crossrail board since taking over the underground.

Questions have persisted about when TfL and the mayor, Sadiq Khan, knew the Crossrail project would be delayed. The announcement came three-and-a-half months before the official opening was due, after admissions of problems with electrification and signalling systems.

Khan has said he was told about the delay days before the public was informed in August. But a TfL board meeting last week appeared to reveal he was told in “June and July about the additional time that would be taken”.

Caroline Pidgeon, the chairwoman of the London assembly transport committee, told ITV News: “I don’t think it’s believable that the mayor of London would not have been briefed that this project was behind time and overspent.”

A spokesperson for the mayor said Khan had been told of increasing cost and scheduling pressures in July, but it was only as a result of an independent review commissioned by TfL, and the 29 August board meeting, that he was informed the schedule could not be met.

The Elizabeth line should eventually increase the capacity for passengers to travel through central London by 10%. The line will link the West End with Heathrow and Canary Wharf, and serve suburbs and towns in the east and west. Once completed, the line is expected to be used by more than 200 million passengers a year.