Former shadow cabinet member Chuka Umunna accused the Labour Party of having a culture of racism and bullying.
In a letter to Streatham residents Mr Umunna urged his old constituents to vote for Liberal Democrats in a move that threatens to overshadow the current candidates.
The 41-year-old was an MP from 2010 until 2019 but is now contesting the Cities of London and Westminster seat as a Liberal Democrat after joining the party in September.
In his letter Mr Umunna wrote: “The Labour Party I joined has let down millions of it’s own supporters by aiding and abetting the Conservatives on Brexit.
“The party has become one that tolerates anti-Jewish racism and a culture of bullying.”
He added: “At the General Election this December, I urge you to vote Liberal Democrats.”
Streatham Labour candidate Bell Ribeiro-Addy defended Mr Umunna’s right to leave the party but disputed claims of racism.
Miss Ribeiro-Addy said: “Chuka Umunna himself is actually a black man that came into politics at a time when there were fewer black politicians.
“The Labour party is pushing for representation and has put forward over 60% women for the first time in their seats and over 30% of BAME people.
“I don’t think that is the record of a racist party and I really don’t like the way that has been played out especially by a person that has left and gone somewhere else.
“I mean he had every right to leave the party absolutely, but I wouldn’t say every right to stay a member of parliament having left the party, but that’s not the discussion.
“If he believed that so much why didn’t he just start as a Liberal Democrat here?” Meanwhile Liberal Democrat candidate Helen Thompson welcomed Mr Umunna’s endorsement.
She said: “Chuka was a very popular MP for the constituency and I know many are sorry to see him move on.
“I’ve talked to lots of local people who no longer feel able to vote for the Labour party under Jeremy Corbyn.”
The Streatham seat, created in 1918 and held by Labour since 1992, voted 79.46% to remain in the EU referendum is also being contested by Conservative’s Rory O’Broin, Green Party MEP for London Scott Ainslie and Brexit Party’s Penelope Becker.
Read more about what’s important to south west London constituencies in our 24-page General Election preview special.