A by-election will be held next month after Reform UK's majority on Durham County Council was cut to just one following the resignation of Aaron Hillam.
Cllr Hillam stepped down from his Craghead and South Moor seat after moving away from County Durham, becoming the latest Reform councillor to leave the authority since the party swept to power in the 2025 local elections.
His departure leaves Reform holding a majority of just one seat, a dramatic reduction from the position it enjoyed after winning 65 of the council's 98 seats in May last year.
The Northern Echo understands voters in the division in the Stanley area will go to the polls on November 19 to elect a new county councillor.
The resignation follows a series of departures from Reform's ranks.
Lyndsey Fox, Nicole Brown and Paul Sexton resigned last month from Reform to sit as independents, leaving amid allegations about the culture within the ruling group, claims strongly rejected by council leader Andrew Husband.
Bishop Auckland county councillor, Lyndsey Fox (Image: Durham County Council)
Despite the losses, Reform has continued to defend its record in County Durham, with party leader Nigel Farage recently telling The Northern Echo he retained confidence in Cllr Husband and describing Durham as a "flagship council".
Nigel Farage has confidence in Durham County Council leader Andrew Husband. (Image: Stuart Boulton)
Read more
Nigel Farage has 'confidence' in Durham County Council leader as majority cut to one
Reform UK can 'take control' of Darlington Borough Council says Robert Jenrick
Nigel Farage shares Reform UK cupcakes and tea with pensioners in visit to Redcar
A Reform UK Durham spokesman said: "Aaron notified us that he has moved away from County Durham and is no longer able to represent his residents to the level he would have liked. Understandably, he has chosen to resign from the council.
"We would like to thank Aaron for his contribution to date and look forward to campaigning for a new, strong Reform representative for South Moor and Craghead."
A Durham County Council spokesperson confirmed Cllr Hillam had resigned and had not been dismissed.