The two leading parties in Chaucer ward have named their candidates for the by-election on Thursday 7 May.
Chaucer ward stretches from St Thomas Street in the north to Elephant & Castle in the south west to Bricklayers Arms in the south east.
Chaucer was a closely-fought ultra-marginal ward at the 2006 and 2010 borough elections, but Labour took all three seats in 2014 with a 170-vote margin between the third and fourth placed candidates.
Southwark Labour has picked Helen Dennis as its candidate.
Dennis – who lives in Peckham and works in Waterloo for Christian Aid – stood for Labour in neighbouring Cathedrals ward last year.
Southwark Labour leader Peter John said: "Helen will carry on the work with Claire has been doing with Labour Councillors Karl Eastham and Vijay Luthra, standing up for local residents, and getting the best for people in Chaucer.
"This May people in Chaucer, Southwark and across the country face a choice about the kind of future they want. It is only with Helen working alongside Neil Coyle, a Labour MP for Southwark that people in Chaucer and across our borough can get a better deal and a fairer future."
Southwark Liberal Democrats have chosen Lawson Estate resident William Houngbo – one of their three candidates in Chaucer ward last year – to contest the by-election.
Lib Dem leader Cllr Anood Al-Samerai said: "William will be a brilliant councillor for Chaucer ward. He lives locally so knows the area, the people and its issues well and he has already worked with Simon Hughes for our community for many years.
"It is a disgrace that Ed Miliband's Labour council has let down local residents so badly by causing an election after less than a year. The fact that Labour have still not given any explanation shows they just don't care and take residents for granted."