Labour MP Heidi Alexander will quit Parliament in order to become one of London Mayor Sadiq Khan's deputies.

The former Labour frontbencher will trigger a by-election in her Lewisham East seat, which she has represented since 2010, after being appointed deputy mayor for transport at the capital's City Hall.

Ms Alexander will replace retiring deputy mayor Val Shawcross this summer and will stand down as an MP before taking up the role.

The ex-councillor served as Jeremy Corbyn's shadow health secretary after he was elected Labour leader in 2015.

However, she joined a mass resignation from Labour's shadow cabinet less than a year later, claiming she "hated" being part of the party's "entirely dysfunctional" top team.


Some big news from me: I have today agreed to become the Deputy Mayor (Transport) for @sadiqkhan. This is a huge job with a massive impact on the life, health and economy of London. I will be leaving Parliament after 8 incredible years. pic.twitter.com/Gz4vxJAkzx — Heidi Alexander (@heidi_mp) May 8, 2018

Since the EU referendum, Ms Alexander has been a prominent pro-EU voice and called for Britain to remain in the bloc's single market and customs union, putting her at odds with Labour policy.

She is co-chair of the Labour Campaign for the Single Market, with Mr Khan having also called for the UK to remain in closely tied to the EU.

Ms Alexander has previously urged Mr Corbyn to condemn Momentum activists seeking to deselect Labour MPs not supportive of their party leader.

She has faced her own battles as a result of Momentum's presence in her constituency.

Commenting on Ms Alexander's new role, Mr Corbyn said: "I would like to thank Heidi Alexander for her service to the people of Lewisham as their MP for the past eight years, and for her work as shadow health secretary.

"I wish Heidi well in her new role as deputy mayor for London in Sadiq Khan's team, where I know she will put her talents and knowledge to great use for the people of London."

Ms Alexander comfortably won her Lewisham East constituency at last year's snap general election, with a 21,000 majority, and Labour will be confident of retaining the seat at a by-election.

She 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."

Image: Ms Alexander quit Labour's 'entirely dysfunctional' shadow cabinet in 2016

Mr Khan thanked Ms Shawcross for postponing her retirement to work with him for the first two years of his administration at City Hall.

He told Sky News: "I want to thank Heidi for accepting my invitation to be the new deputy mayor for transport.

"She is massively experienced, as a senior parliamentarian, as a former councillor and a deputy mayor in Lewisham.

"I'm sure she'll do a great job filling the big shoes Val leaves behind."

Ms Alexander joins other prominent Labour MPs Andy Burnham and Steve Rotherham in quitting Parliament under Mr Corbyn's leadership in order to take up top roles at city authorities.

Last week, Dan Jarvis was elected the first mayor of the Sheffield City Region but will remain as Barnsley Central MP after overturning an initial decision by Labour's ruling body that he could not hold both jobs.

Ms Alexander's appointment is subject to a confirmation hearing by the London Assembly.

Newly-elected Lewisham councillor Sakina Sheikh, a strong supporter of Mr Corbyn, immediately put herself forward to be Labour's by-election candidate.

Responding to the news of Ms Alexander's resignation, Conservative Party chairman Brandon Lewis MP said: "After a dismal set of local election results in which Labour failed to make the gains they boasted about, this is more proof that Labour are too divided to deliver on their promises to working people.

"This also highlights a complete lack of faith in Jeremy Corbyn and his leadership.

"The remaining moderates left within Labour will now rightly be concerned that they are next on Momentum's list."