Former Conservative MP Steven Norris is taking over as interim chairman of outsourcing firm Jarvis.
Paris Moayedi: Leaving Jarvis after 9 years
Mr Norris replaces Paris Moayedi, Jarvis' flamboyant Iranian-born boss.
Mr Moayedi, who joined Jarvis as chief executive in 1994, is credited with turning the firm into one of the UK's biggest engineering contractors.
His resignation comes as Jarvis faces an investigation into its maintenance of the railway track at Potters Bar, scene of a fatal crash last year.
Separately, Jarvis announced pre-tax profits of £33.7m for the six months to September, a 77% increase on the same period last year.
Turnover was also sharply higher, climbing 24% to £710m.
The increase partly reflected new revenues from Jarvis' stake in Tube Lines, the consortium reponsible for upgrading London Underground's Piccadilly, Northern and Jubilee lines.
Mr Norris, the Conservative candidate for the post of London mayor, is expected to quit Jarvis in order to avoid conflicts of interest if his mayoral bid is successful.
The mayor's duties include monitoring the performance of private sector contractors engaged to maintain the capital's transport network.
But the Liberal Democrats' mayoral hopeful Simon Hughes said Mr Norris' role as Jarvis chairman was incompatible with his campaign to become mayor.
"It will prevent Mr Norris making any objective comments on any Tube issues from now on, despite this being one of the key responsibilities of the London mayor," he said.
"He must now decide which job he most wants to do."
Mr Moayedi said now was a good time to step down as he had completed the process of handing over to the company's new chief executive Kevin Hyde.
"It has been a privilege for me to lead Jarvis over the last nine years," he said.
Jarvis said Mr Moayedi had waived his right to bonus payments for the current financial year, and would be compensated in line with his 12 month notice period.
Last month, Jarvis decided to abandon the railway maintenance business amid growing criticism over the standard of work carried out by private contractors, citing "reputational issues".
Jarvis, which is also involved in building and maintaining schools, hospitals and roads, said it now planned to move into rail freight haulage.
Jarvis shares closed 4.8% lower at 222.75p as investors questioned the company's long-term strategy.
"It's not quite clear why they suddenly want to go into new areas, and whether they've got a chance of being very successful in there," said Geoff Allum at Investec Securities.