Hampstead Town's first ever Labour councillor is to stand down just three weeks into office.
Cllr Adrian Cohen has said he's "deeply sorry" but "personal circumstances" mean that he can't take up the seat.
Plans are being put in place for a by-election in the summer.
Cllr Cohen was awoken early in the morning during the election count on May 6 and urged to join when it looked like he was about to win the seat that was held by the Conservatives.
But nearly three weeks after being elected, he has announced that he will be stepping down.
In a statement he said: "I would like to thank the residents of Hampstead Town for electing me to be on of their two councillors representing the ward.
"This historic victory for Labour was truly unexpected.
"I am grateful to everyone who gave me and my fellow candidate Alexandra Sufit their support and who felt, like us, that the Conservatives both locally and nationally have let us down badly."
He added: "Unfortunately my personal circumstances now mean I cannot take up the seat as a Hampstead Town councillor.
"I am deeply sorry that I cannot do justice to your support, but Hampstead Town deserves a Labour representative who can work every single day for our community.
"I can't do that and it would be wrong for me to serve as your councillor when I know I cannot give you the attention and dedication the job needs - consistent with my values and the values of Camden Labour."
The Labour Party won 47 seats following the election, while the Liberal Democrats took one in Fortune Green and three in Belsize Park, where Conservative group leader Oliver Cooper failed in his challenge, having moved from Hampstead Town.
The Tories took two seats in Frognal and one in Hampstead Town, down from the seven total they previously had, and the Liberal Democrats became the official opposition.
Cllr Cohen added: "Going forward I will be supporting a Labour candidate who can be the dedicated champion that Hampstead needs."
Former Hampstead Town councillor, Oliver Cooper said: "It's been a real honour to represent Hampstead for the last seven years and flattered that people have asked me to stand again.
"In Stephen Stark, Hampstead has the hardest working councillor in Camden.
"But it needs another to work with him to stand up for Hampstead and solve problems locally.
"I can't now give Hampstead the time that it has come to expect thanks to the hard work of Conservative councillors like Stephen Stark over the years, but I look forward to backing a candidate who will work with Stephen to defend what makes Hampstead special."
A Camden Council spokesperson said: "Following the resignation of Cllr Adrian Cohen a By-Election will be held in the Hampstead Town ward.
"The date of this By-Election will be confirmed in due course."
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