Conservative county councillor Maureen Hill has quit her party and joined the Liberal Democrats following a fall-out over the authority's budget.
Councillor Hill, who represents the Weston ward on Northamptonshire County Council, officially announced her decision yesterday.
It came after she wrote a letter to the leader of the authority, Councillor Jim Harker (Con, Kettering Rural) saying she was "increasingly at odds" with the Conservative administration.
During a press conference to announce her decision, she said: "I think it's been a natural progression for me. I've found myself so much at odds with the policy of the administration I don't feel I could possibly support any further budget cuts."
Councillor Hill said another reason for her decision to quit the Conservatives was the handling of the controversial Northampton school site sell-off plan.
The leader of the Liberal Democrat group on the county council, Councillor Brian Hoare (Lib Dem, Abington) said: "Maureen has made many contributions in the chamber that have echoed our thoughts, so we're delighted to have her join us."
Councillor Hill's move will leave the Liberal Democrats at County Hall with nine members.
The move has been criticised by Councillor Hill's former Conservative colleagues.
But Councillor Harker said the move was not a shock, adding: "It comes as no surprise to us as Councillor Hill has not supported us for a long time."