A vote to choose a new leader of Lewisham will be fast-tracked after the borough's mayor quit to fight the Kingswood by-election on Wednesday morning.
Damien Egan has been announced as the Labour candidate for ex-Tory minister Chris Skidmore's seat.
Mr Skidmore quit as an MP this week in protest at Government legislation on issuing new oil and gas drilling licences in the North Sea.
It is another headache for Mr Sunak after disgraced Conservative MP Peter Bone was kicked out by his Wellingborough constituents in December, sparking a by-election there.
Dates for the upcoming votes are set to be confirmed this week, but a London Labour spokesman said the Lewisham poll will be a "truncated process".
Mr Egan who grew up around Kingswood, near Bristol, was first elected as a councillor 14 years ago and has been Mayor of Lewisham since 2018. Labour won all 56 seats on the council in May 2022.
Mr Egan said: “On Friday the current MP for Kingswood, resigned. I have been selected to fight this by-election and therefore I have informed the [council's] Chief Executive of my decision to resign as the Mayor of Lewisham.
“I want to thank everyone in Lewisham for their support over the last 14 years from when I was first elected as a Lewisham councillor to now as Mayor.
"It has been an honour and a privilege to be the Mayor of Lewisham and I am proud of what we have achieved by working together: turning around Children’s Services, improving local schools, an ambitious council house building programme, and our work to support independent high street businesses.”
At the 2019 election Mr Skidmore won the Kingswood's seat for the Tories with a majority of more than 11,200 votes over second-placed Labour.
Mr Egan added: "Speaking to residents in Kingswood today, it’s clear that just like the former MP, people are fed up of this government and ready for change."
