Labour MP Heidi Alexander is quitting Parliament to work for London Mayor Sadiq Khan.
The Lewisham East MP will become the new deputy mayor for transport.
It entails there will be a by-election in her South London seat, where Labour won by more than 20,000 votes in last year’s general election.
Announcing the appointment, Mr Khan said Ms Alexander – who replaces retiring Val Shawcross – was “respected across the political divide”.
Ms Alexander said she was “really excited” at her appointment, adding: “I know how important it is we ensure everyone has access to a high-quality and affordable public transport network, with safe cycling routes across the capital.”
She has been an MP since 2010, and was shadow health secretary between 2015 and 2016, resigning from the front bench with an attack on Jeremy Corbyn’s leadership.