Lee Anderson hailed a “seismic moment” as the first Labour councillor defected to Reform UK.
Cathy Mason, who sits on Ashfield District Council, said she was “disappointed” with Sir Keir Starmer’s policies including stripping most pensioners of winter fuel payments.
She becomes the town hall’s third Reform councillor and her switch leaves Labour with no representatives.
Mr Anderson, the Reform MP for Ashfield, said: “This is a seismic moment for Reform UK, the first Labour councillor has defected to us today and it’s happened in the capital of common sense, Ashfield.
“Cathy will be the first of many to join Reform UK from Labour and she will be one of millions to leave Labour to vote for Reform UK at the next general election. We are the only party that can beat Labour.”
Ms Mason, who represents the Carsic ward, added: “I am personally disappointed that the Labour Party’s policies since the election have not helped people and going forward will not help the people of Ashfield.
“Residents are suffering and they need change, Reform is now the only party offering this.”
Her move follows last week’s defection of Ashfield Independent councillor Andy Gascoyne to Reform UK.