Hopgood sent a petition to residents in Benfieldside calling for Reform UK to cover the cost of the election, according to the Local Democracy Reporting Service.
She said: “Reform have made a huge issue of reducing council ‘waste’, but there can be few things more wasteful for the taxpayer than calling a by-election because they could not get their paperwork in order.”
Despite the setback, Reform UK said it hoped to retain its stronghold on the ward, which also covers Blackhill and Shotley Bridge.
The party secured a landslide victory just weeks ago to control the council, with its 65 councillors officially inducted earlier this month.
A Reform UK spokesman said: “We understand that Mr Kilburn did not declare his employment to the returning officer as expected and is now required to resign his seat.
“We look forward to contesting the upcoming by-election to give the people of Benfieldside a strong voice in Durham County Council.”
A full list of candidates for the July by-election will be released following the nomination deadline on Friday, 6 June.
