Southampton will be run by the Conservatives for the first time in 23 years after a Liberal Democrat councillor broke ranks.
Cllr Norah Goss said she would continue to vote with the Lib Dems
The shock move occurred as councillors gathered to elect a new leader.
The Tories and Labour both have 18 councillors following this month's local election, the Lib Dems have 12.
But on Thursday, Lib Dem councillor Norah Goss split with her party to vote in a Tory administration while her former colleagues abstained.
Cllr Goss, a former city mayor who was first elected in 1988, said: "When we (Lib Dems) took over the council we worked hard to get up to a three star rating and we were a notch away from being rated excellent.
"I did not want that being squandered.
"I've not had any conversations or meetings with the Conservatives and this is nothing against the Liberal Democrats.
"I'd like to go back but I understand the party needs discipline. I'll still be voting with the Lib Dem group."
A council spokeswoman called the move a "shock".
As a result, Tory leader councillor Alec Samuels was elected council leader and Cllr Goss will act as an independent.
Meanwhile in Portsmouth, the Lib Dems' 19 councillors have gained the backing of Labour's five councillors to form a coalition administration. The Tories have 17 councillors.