The Fianna Fáil chief whip says Brian Crowley has put himself outside the parliamentary party by leaving their grouping in the European Parliament.
The MEP announced yesterday he was joining the same grouping as Britain's Tories and a number of Eurosceptic parties.
Members of Fianna Fáil's parliamentary party will discuss his move at a meeting today.
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.