UKIP has lost overall control of the only local authority it runs following the defection of one of its councillors to the Conservatives.
The loss of Beverly Martin means the party only holds 27 seats on Thanet District Council while the other parties combined hold 28.
Ms Martin said she joined the Tories after UKIP failed to make "significant change".
UKIP party members have condemned her decision to leave.
UKIP councillor Stuart Piper said: "I don't think anyone could doubt her care and concern for Ramsgate as a town, but in a sense she's just thrown away the only job she had to coordinate town promotion."
In October 2015 five councillors defected from the UK's first UKIP authority due to concerns over the council's lack of action over Manston Airport.
However, the party regained control in 2016 following a by-election.
Ms Martin has swapped allegiance twice before - from UKIP to DIG Alliance and then back to UKIP.
Speaking about this occasion, she said: "There has to be a very good reason for making a political change.
"The first one when I became an independent was specifically on the issue of Manston.
"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. Frankly we haven't, not as UKIP.
"It's Craig Mackinlay, our MP for South Thanet who invited me last year to form a group for assessing what we might do with the port and the beaches, that came from the Conservatives.
"That is where the energy is coming from."
In the June general election UKIP's vote fell by 26.4% in South Thanet and 21.2% in North Thanet - leaving them with just 6% and 4.5% of the vote.
Rajdeep Sandhum - Political Reporter, BBC Radio Kent
First the local elections and the party loses every seat, then the General Election and it fails to win any seat. Now it's lost the only council it controls in the country. It's been a rough ride for the party's councillors and members. They still don't know who'll lead the party nationally.
Councillor Beverly Martin says she's always been a Conservative at heart and that joining the group felt like a "homecoming". But what of the voters? Did they know that underneath the purple rosette was a blue one?
UKIP group leader Chris Wells told me wasn't surprised by the news, which he heard first from press. He said they'd been there before and they'd get through it again. This isn't the first time he's lost his majority. Nor is it the first time she's left UKIP.
There will undoubtedly, as there always is, when a politician changes sides be calls for a by-election. But it doesn't sound like Councillor Martin will step down before the election in two years time. She says she'll still be fighting for the things she stood for.