Ukip has lost control of the last local authority it had a majority on, after a councillor defected to the Conservatives.
The party had previously held 28 of 55 seats on Thanet district council, but after Beverly Martin crossed the floor, it is now outnumbered.
A coalition of Conservative, Labour and Independent councillors can now defeat the party if they work together.
Ms Martin told the BBC she decided to leave Ukip after the party failed to make “significant change”.
“We had the opportunity to be a flagship council, that is a very rare privilege and I really had enormous hopes that we would make significant changes on social issues, development and economic issues,” she said. “Frankly, we haven’t, not as Ukip.”
The politician also said in a statement that local issues reinforced her decision.
“When Ukip stood for election 2015, we majored on the pledge to re-open Manston as an airport, and to secure a CPO at the earliest opportunity. This did not happen and Manston remains closed,” Ms Martin said.
She also accused members of “completely ignoring” a campaign to give support to the iconic P22 US Gunship, which is featured in the recently-released film Dunkirk and said she has “serious concerns that Ukip intended to promote and develop Ramsgate as an industrial town”.
It’s the latest of a series of blows to the party, after it lost every seat it was defending and won only one other in May’s local elections.
Ukip saw its vote share fall below two per cent in the general election, tumbling from 12.6 per cent in 2015.
The party is in the midst of its fourth leadership campaign in just over a year after Paul Nuttall stepped down.
The politician quit as Ukip’s leader within hours of the party’s dismal election performance.
His deputy Peter Whittle is currently running for the position against race against Sharia Watch UK director, Anne Marie Waters, who has called Islam “evil”.
Former Ukip leader Nigel Farage ruled out the possibility of a comeback, announcing earlier this month that he would not be making a bid to return to the role.
The three-time party leader cited the threat of violence and the “amateur shambles” of the party’s national executive committee as primary factors in his decision.
