Labour could lose control of Brighton and Hove City Council after one of its councillors joined the Conservatives in protest at anti-Semitic abuse.
Anne Meadows, whose husband is of Jewish descent, said she had suffered 18 months of attacks.
“There has been a lot of anti-Semitism going on in the city and that was aimed at me as well,” said Ms Meadows.
Last July, she was deselected by Labour party members ahead of this year’s local elections.
The Tories are now the council’s largest party and will try to form a minority administration.
Ms Meadows, who represents the Moulsecoomb and Bevendean ward, said the abuse “came to a head even in Labour group meetings where I would be attacked, and none of the other councillors would prevent it or halt it.
“I used to go home feeling awful and I thought there’s got to be more than this, it’s not the Labour party I knew and loved.”
Conservative Party chairman Brandon Lewis tweeted that he was delighted to welcome her to the party.
He said she had been “driven out by the disgraceful anti-Jewish racism that runs through Jeremy Corbyn’s Labour Party”.
The Conservatives were last in charge of the council eight years ago.
The defection of Ms Meadows, 61, follows the resignation of eight Labour MPs, citing Labour’s failure to tackle anti-Semitism and Mr Corbyn’s leadership.
Responding to the decision, a regional Labour spokesperson said: “We are very disappointed to learn that councillor Anne Meadows has taken the decision to defect from Labour to the Conservatives.
“We want to thank Anne for her years of dedication and hard work.
“The Labour Party is committed to tackling anti-Semitism in all its forms wherever it arises, in our party and wider society.”