Labour's newest MP Janet Daby vowed to oppose "hard Brexit" after winning the Lewisham East by-election.
Only a third of eligible voters cast their ballot as Ms Daby held the seat with a majority of 5,629, well below the 21,213 Heidi Alexander achieved at the 2017 general election when turnout was 69%.
The Liberal Democrats leapfrogged the Conservatives to take second place in a seat which voted heavily for Remain in the European Union referendum.
Ms Daby said the result showed "we will not tolerate an extreme Brexit in Lewisham East".
Explaining what she had found on the doorstep during the campaign, she told reporters: "One of the ongoing issues is to do with Brexit and people not wanting an extreme Brexit and ideally people not even wanting to leave the EU.
"I will oppose an extreme or a hard Brexit.
"I will consider people's jobs, the economy and people's rights and that will dictate to me the precedent in how I will conduct myself both within Parliament and out of Parliament."
Jeremy Corbyn suffered a revolt in the Commons after ordering Labour MPs to abstain on the issue of Norway-style single market membership this week.
Asked how much distance there was between her and Mr Corbyn's views on Brexit, Ms Daby said: "In terms of the single market, our Labour view on this is that we would have a new single market where we have the same access that we presently do.
"For me it's about ensuring that we have as close a relationship to the EU as possible and that's what I'll be arguing for and that's what I'll be putting forward my views on.
"In terms of Jeremy Corbyn I voted for Jeremy Corbyn twice in the leadership election and obviously within politics not everybody will agree on everything... but we need to find a way around it that holds the Tories to account."
Ms Daby picked up 11,033 votes - a 50.2% share, down nearly 18 percentage points from Ms Alexander's vote share in 2017.
Lib Dem Lucy Salek had 5,404 votes, a 24.59% share, while the Conservative Ross Archer had 3,161 votes, a 14.38% share.
The result put the Lib Dems up more than 20 percentage points, with a swing of nearly 19% from Labour to Lib Dem.
Ms Salek said the result showed the party was "the real opposition here".
She said: "We have turned around a massive increase in our vote share. We're back and people are starting to notice.
"This is about recognising that the people have a final say on Brexit."
But Ms Daby said: "I'm very aware the Lib Dems pitch themselves as being the party to oppose Brexit and my thoughts are that's probably quite misleading because they don't have much bite in Parliament.
"It was a way in which they could find a niche to try and make themselves as a reputable party."
Lib Dem leader Sir Vince Cable said: "This result sends a message to the Labour leadership that it cannot take pro-European voters for granted.
"This is the largest swing from Labour to the Liberal Democrats in over a decade and the failure of Labour's leadership to oppose the Conservatives' hard Brexit cannot be hidden or forgotten.
"The collapse in the Conservatives' vote is also damning."