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Chuka Umunna will try to oust the Conservatives from their flagship seat that covers the City, Buckingham Palace and 10 Downing Street, the Standard can reveal.

In an exclusive interview, the Remain campaigner announced plans to stand in The Cities of London & Westminster with a campaign billed as a mini-referendum on Boris Johnson’s threat of a no-deal Brexit.

Sitting Tory MP Mark Field had a reduced majority of 3,148 at the 2017 election in the constituency where 72 per cent of residents voted Remain in 2016.

Mr Umunna said: “I worked as a solicitor in the City and West End and I know they are the last places that should be represented by one of Mr Johnson’s MPs who has just voted to enable a no-deal Brexit.

“The City is the centre of international values, while the West End is home to our greatest creative industries. These centres are telling us that Brexit will be bad for them.”

Mr Umunna, who joined the Liberal Democrats in June and is now their foreign affairs spokesman, quit the Labour Party in February in dismay at anti-Semitism and Jeremy Corbyn’s mixed messages over Brexit.

He said he was “sad” to leave Streatham which he has represented since 2010 but would not stand under his new colours because the Lib Dems already had a candidate.

Mr Field is a staunch Remainer at heart but Mr Umunna said the MP had supported the Prime Minister’s attempts to leave on October 31 in this week’s Commons showdowns.

“He should have stood with Kenneth Clarke, Dominic Grieve and the rest of the 21 Tory rebels,” he said.

The Lib Dems came a poor third behind Labour and the Tories in the seat in 2017 but recent local and European voting showed them overtaking both parties.

In June’s Euro-elections they came top, with 40 per cent in the City of London and 33 per cent in Westminster.

Mr Umunna denied it was mission impossible, saying the European results had been “a comfortable win” for his new party.

“I will fight tooth and nail to take this battle to Mr Johnson and his ‘Vote Leave’ government and ensure it is represented by a Remainer committed to stopping Brexit,” he said.

“I resigned from Labour primarily because of the party’s failure to oppose Brexit. Whilst it is important to win seats from Labour, it is also vital we win seats from Mr Johnson’s populist nationalist Tory party in order to stop Brexit and build a more liberal and inclusive Britain.

Mr Umunna’s wife Alice is an employment lawyer. He added: “My family is absolutely delighted that I will be taking the fight to this constituency which has been such a big part of our lives.”

Mr Field hit the headlines in June when Greenpeace accused him of heavy-handed behaviour when a female activist gatecrashed Philip Hammond’s Mansion House Speech.