Labour MP Heidi Alexander is to stand down from Parliament after being confirmed as London's deputy mayor for transport.
Ms Alexander's departure will trigger a by-election in her Lewisham East constituency in south London, a safe Labour seat where she secured an overwhelming majority of 21,123 in the 2017 general election.
The former shadow health secretary, who entered Parliament in 2010, was one of a large number of Labour frontbenchers who quit roles in Jeremy Corbyn's team in 2016.
She will replace current deputy mayor Val Shawcross, who is stepping back from full-time public life after 18 years at the Greater London Authority, the office of Mayor Sadiq Khan has announced.
Ms Alexander, 43, said: "After eight years as the Member of Parliament for Lewisham East and six years as a local councillor, I know just 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.
"London is a fantastic city. I know Sadiq wants its transport system to be the envy of the world and I am looking forward to playing my part in making that happen."
Mr Khan thanked Ms Shawcross for her "exceptional service" and said that Ms Alexander - who takes over as deputy mayor in the summer, subject to confirmation by the London Assembly - is "respected across the political divide as a parliamentarian and campaigner, and Londoners can be confident she will ensure we deliver our ambitious plans to transform London's transport network over the coming years".
Since quitting the Labour frontbench, Ms Alexander has campaigned against a hard Brexit, tabling a motion to stop the Article 50 letter triggering the UK's EU withdrawal.