Sir James Cleverly is stepping down from the shadow cabinet to stand for London mayor.
The former foreign secretary and Braintree MP said the capital needed "a champion, someone who understands its potential and is prepared to fight for its future".
Sir James - who has served as shadow housing secretary since July 2025 - revealed on Saturday that he had told Conservative leader Kemi Badenoch of his decision to stand down.
Writing for The Sunday Times, he said: "We need a credible big hitter putting themselves forward for Mayor of London."
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The former minister said he had stepped down to "concentrate fully on winning this race".
He added: "I have told Kemi Badenoch that I will leave the shadow cabinet. Serving there has been a privilege, and I know there will continue to be strong voices around that table.
"But to run the most effective campaign possible, I must be free to speak about every aspect of London's future and concentrate fully on winning this race."
In a post on X, he said: "London needs more than an administrator. It needs a champion, someone who understands its potential and is prepared to fight for its future.
"That's why I'm running to be the Conservative Candidate for London Mayor."
Sadiq Khan has served as mayor of London since 2016 and is yet to confirm whether he will run for a fourth term.
The next mayoral election is scheduled to be held in on 4 May 2028.
(c) Sky News 2026: Conservative MP James Cleverly quits shadow cabinet to stand for London mayor
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