The move has reduced the ULA's representation to just three TDs -- with Higgins now effectively an independent deputy in the Dail.
The outspoken Dublin West TD took a number of parting shots at his former pals, including Clare Daly and Joan Collins.
In a statement, his Socialist Party accused the other ULA TDs of having "moved away from a principled Left position" as well as having "ditched the collaborative spirit".
The party also accused People Before Profit TD Joan Collins and former Socialist Party member Clare Daly of "consciously" choosing not to campaign on issues under the ULA banner.
But sources pointed to the support the pair have shown to Wexford TD Mick Wallace in recent months.
A number of former members of the ULA are understood to be considering forming a new alliance, which will exclude Mick Wallace.