Fianna Fáil's only remaining MEP has announced he's changing his group in the European Parliament.
Brian Crowley is to leave the ALDE group and join the ECR - the Eurosceptic group set up by David Cameron's Conservatives five years ago.
The move is a blow to Fianna Fáil leader Micheal Martin, who is understood to be deeply unhappy with the move.
Crowley's status within Fianna Fáil is now unclear, as Fianna Fáíl itself is still a full member of ALDE.
Fianna Fáil said the decision will be discussed at the Parliamentary party tomorrow.
A statement issued a short time ago describes Mr Crowley's actions as "unacceptable, particularly given the fact that Brian successfully ran for election under the ALDE banner."
"We are disappointed with Brian's personal decision to join the ECR and by doing so align himself to policies and political parties that directly contradict core principles of the Fianna Fáil organisation," party officials said.