HS2 and Crossrail chairman Sir Terry Morgan has resigned, the Department for Transport has announced.
The announcement comes days after Sir Terry said he expected to be sacked and just months after he was appointed to the HS2 role.
He told the BBC at the weekend that there had been disappointment in Westminster about delays affecting London's £15bn Crossrail project, which he has chaired since 2009.
The east-west railway was due to open this month but its launch has been pushed back until at least autumn next year due to delays in completing infrastructure and testing.
Its budget was raised from £14.8bn to £15.4bn in July due to "cost pressures" - and in October the mayor of London was offered a £350m loan to cover costs caused by delays to the project.
The departure of Crossrail chief executive Simon Wright was announced last month.
Sir Terry became chairman of HS2 in August but there has been growing speculation that the high-speed railway will exceed its £55.7bn budget.
Transport Secretary Chris Grayling said: "Sir Terry has been an integral part of Crossrail for almost a decade and I would like to thank him for his dedication and the expertise he brought to the role.
"I am also grateful to him for his work as chair of HS2 Ltd."
Mr Grayling added: "HS2 is the country's biggest infrastructure project and, with his wealth of experience, Allan Cook is the right person to oversee the project as it progresses towards full construction."