Fianna Fáíl's only MEP could face suspension from the parliamentary party today after leaving their grouping in the European Parliament.
Brian Crowley has joined the same group as David Cameron's Conservatives and some other Eurosceptic parties.
Mr Crowley says he joined because the ECR is a "non-federalist, pro-EU group" which wants national governments to have more power, and because it means he can save the jobs of his Irish staff who could be laid off otherwise.
However, Fianna Fáil says his staff would have kept their jobs anyway.
The party says it is "unacceptable" for someone who ran under the Fianna Fáil banner last month to now join a group whose principles openly conflict with its own.