Left-wing TD Paul Murphy has quit the Socialist Party and plans to launch a new political grouping next week.
The Dublin South-West TD's surprise departure from the Socialist Party and its Solidarity offshoot marks a further fracture in left-wing politics.
It comes amid disagreement in the Socialist Party over the extent to which it should work with other left-leaning parties such as Sinn Fein and the Greens, as well as broader left-wing campaigns.
Mr Murphy favours working more closely with these parties and movements, but this is opposed by the majority of Socialist/Solidarity members.
Mr Murphy will announce plans on Monday for a new socialist grouping that will remain part of the Solidarity- People Before Profit grouping in the Dail, but aims to work more closely with other left-wing parties.
He insisted that his relationship with the two other Socialist/Solidarity TDs - Ruth Coppinger and Mick Barry - remains "amicable and co-operative" and he will continue to work with the Solidarity-PBP group.
The group includes PBP TDs Brid Smith, Gino Kenny and Richard Boyd-Barrett.
"After an extensive debate within the Socialist Party, a group of members, including me, have decided to leave and intend to launch a new socialist group," he said.
"When we decided to leave, the party accepted that and we all aim to have an amicable and co-operative relationship in the future in the interests of working class people."
Several Socialist/Solidarity activists, but no elected representatives, will join Mr Murphy in his new grouping.
He was first elected to the Dail five years ago off the back of the anti-water charges campaign.