It is claimed as many as 50 will not serve under the shadow business secretary and would instead stage a walkout, according to party insiders.
The split could see some continue as independent MPs but refuse to take the Labour whip while others may quit parliament altogether.
While the bloc has not met formally to discuss a co-ordinated move, HuffPost UK has spoken to numerous sources who say they have an understanding with others.
One MP said she “would definitely go” and a “conservative estimate” of the number of MPs that would follow was between 30 and 50.
Another MP named a specific figure of 35, and added: “The Labour Party would be sticking two fingers up to the public to stay on the same course as set by Corbynism. MPs won’t go through that again and the public won’t stand for it.”
Bermondsey and Old Southwark MP Neil Coyle, who put the number “closer to a dozen”, told HuffPost UK: “Continuity Corbyn is a death knell for Labour.
“Division, factionalism and introspection will continue. Favouritism and bullying will continue. Anti-Semitism will continue. Failure in elections will continue.
“More MPs and others will leave. The frontbench will continue to fail to reflect members. It’s a recipe for disaster.”
The news follows a turbulent period in the Parliamentary Labour Party (PLP) during Corbyn’s leadership, where a number of MPs defected to the Lib Dems and to sit as independents.
Others, however, say the mood among MPs is generally good with most optimistic about a new era.
Ilford North MP Wes Streeting said he was sceptical any breakaway would materialise given the failure of the Independent Group, and that MPs saw shadow Brexit secretary Keir Starmer as the most likely winner not Long-Bailey.
“People should be campaigning to win, not preparing to fail,” he said.
Long-Bailey is battling it out for the top job with Starmer, campaigning Wigan MP Lisa Nandy and shadow foreign secretary Emily Thornberry for the top job.
Starmer is currently the bookies’ favourite, with the backing of large unions such as Unison, a huge chunk of MPs and the highest number of local parties swinging behind his campaign.
Lisa Nandy has also built up a bedrock of support by orienting her offer around towns, largely in the north and midlands, which Labour lost at the election.
But with the backing of Momentum and key left-wing trade unions such as Unite and CWU, Long-Bailey is also in a strong position.
It looks highly unlikely that Thornberry will make the final ballot, however, having failed to secure enough backing or enough local parties to make the cut.
A source close to Long-Bailey said the leadership candidate had committed to building an inclusive team.
They added: “Rebecca has committed to appointing a shadow cabinet that draws on the experience of all parts of the Labour Party, and she is committed to all Labour MPs unifying behind a message that can repay the hard work of party activists and win the next general election.”
The race is decided on a ballot of members by a proportional representation system known as AV (alternative vote), which sees candidates ranked in order of preference.
If one of the five wins more than 50% of first preference votes, they are declared the winner outright. However, if no candidate hits 50%, the candidate with the least support is eliminated and their second preferences are allocated to the remaining contenders. That process is repeated until one candidate reaches that crucial 50% mark.
The winner is announced at a special conference on April 4.