Labour has fast-tracked the selection process of choosing the candidates it hopes can oust the MPs who quit the party on Monday.
Ian Lavery, the Labour chairman, has written to party members in the seven constituencies held by the splitters.
The process of selecting a candidate to challenge Chuka Umunna in Streatham will begin next week.
In a letter to Labour members in the south London seat, passed to HuffPost UK, Lavery said Umunna had “let down” his voters.
He said the former party leadership hopeful was only able to increase the Labour vote by 13.2% in 2017 due to the “hard work” of local activists “taking Labour’s positive message to the voters”.
Umunna has a majority of 26,285 – up from the 13,934 majority he won in 2015. This morning he told ITV he hoped The Independent Group of MPs he is spearheading would have evolved into a new party “by the end of the year”.
In his letter, Lavery said: “Our Labour majority increased because voters wanted to see the changes promised in Jeremy Corbyn’s manifesto. Because Labour represents hope for the many.”
“Yesterday Chuka Umunna decided to resign from the Labour Party.
Chuka’s resignation has let down the people of Streatham who wanted to be represented by a Labour MP. You worked hard to get Chuka elected, and I understand many members will feel very let down too.
“As the 2017 election showed us, when we are united, when we have the policies that resonate with the electorate, we are at our strongest.
That’s why next week we will begin the process of selecting a new candidate, someone who will fight for Labour’s ideals on behalf of the people of Streatham.”
Not all the MPs who quit Labour yesterday have as healthy a majority as Umunna.
Angela Smith, the MP for Penistone and Stocksbridge, beat her Tory challenger in 2017 by 1,322 votes, down from 6,723 in 2015.
The Labour leadership has demanded Umunna, Smith, Chris Leslie, Luciana Berger, Mike Gapes, Gavin Shuker and Ann Coffey submit to immediate by-elections.
Momentum, the pro-Jeremy Corbyn grassroots campaign group, is already making plans to campaign locally in each seat.