Dublin South-West TD Paul Murphy says he has left the Socialist Party after "extensive debate".
He's to launch a new group on Monday, along with a number of other members.
In a statement, he says: "When we decided to leave, the party accepted that and we all aim to have an amicable and cooperative relationship in the future in the interests of working class people.
After an extensive debate within the Socialist Party, a group of members, including me, have decided to leave and intend...
"I will continue as a member of Solidarity-People Before Profit and all six TDs in Solidarity-PBP will continue to work together closely in Dáil Éireann and in building and assisting struggles in workplaces and communities".
Deputy Murphy is a TD for Dublin South-West, having been first elected in a by-election in 2014 and re-elected in 2016.