The so-called 'Bersanians' criticised Puglia Governor Michele Emiliano for deciding to stay in the PD despite his criticism of ex-leader Matteo Renzi, who he will challenge in upcoming primaries.
The PD split has long been brewing and came to a head on Sunday, when the ex-premier quit as leader to force a fast track to a congress, ignoring rebels' pleas for a longer process.
Mr Bersani, Mr Speranza and others, including Tuscany Governor Enrico Rossi, have long criticised Mr Renzi's shifting of the party towards the centre and his allegedly high-handed treatment of dissenting voices.
More recently, they have criticised his alleged lack of reflection after a crushing December defeat in a constitutional reform referendum, which led to his resignation as Italian premier.
According to pollsters, the splinter group would get between 5 per cent and 8 per cent in a future general election.
The rebels have said they will continue to back Premier Paolo Gentiloni, the former foreign minister in Mr Renzi's government.
With ANSA
