Fianna Fáil has said the decision by its only MEP to leave the party’s grouping in the European Parliament to join a Eurosceptic alliance led by Britain’s Tory party is “unacceptable”.

Brian Crowley topped the poll in Ireland South in last month’s European elections and had previously been aligned with the Alliance of Liberals and Democrats for Europe (ALDE).

Senior Fianna Fáil sources say his decision today to join the Tory-led European Conservative and Reformist (ECR) group was Mr Crowley’s alone and does not reflect a position by the party to move away from ALDE.

Mr Crowley’s move is in open defiance of party leader Michéal Martin, but sources said it is “not as simple” to suggest that the MEP will now lose the party whip.

In a statement this afternoon, Fianna Fáil described Mr Crowley’s move as “unacceptable” and said the party remains a committed member of ALDE.

However, it did not suggest Mr Crowley will be subject to any disciplinary action.

The joint statement from chairman Brendan Smith and Chief Whip Seán Ó’Fearghaíl described Mr Crowley’s new grouping as Eurosceptic, adding: “A number of the parties in this right wing grouping stand in direct opposition to Fianna Fáil’s republican ethos and tradition”.

“This is unacceptable, particularly given the fact that Brian successfully ran for election under the ALDE banner,” the statement says.

“Fianna Fáil remains a committed member of the ALDE party and Brian Crowley’s decision to leave will be discussed at the weekly Fianna Fáil Parliamentary Party meeting in Leinster House tomorrow.

“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. These include the group’s eurosceptic stance and its position on membership of the Eurozone.”

In a statement, Mr Crowley said the change will allow him “provide a better service for my constituents”.

“The ECR Group is a non-federalist, pro-EU group who believes in giving a much stronger say to national governments rather than to EU Institutions,” he said.

“I also made this move to protect some jobs of Irish staff in Brussels, and not for any personal gain for myself, except my new group will allow me work more effectively for my constituents in Ireland South.

“The ECR Group is now the third biggest group in the EU and its priorities include creating long-term jobs and prosperity for member countries, which matches my policy priority.”

Mr Crowley was never particularly close to ALDE. He is understood to have been approached in recent times by a number of rival groups, among them the ECR. He was also approached by factions within the centre-right European People’s Party, which is Fine Gael’s European affiliate.

After Fianna Fáil’s loss of seats held by Pat the Cope Gallagher and Liam Alyward in last month’s elections, Mr Crowley is the party’s sole representative in the European assembly. At issue in the haggling with ALDE was his insistence on his right to retain two Irish staff on his parliamentary team. “If the Irish staff would be properly accommodated for, that’s where I will be,” Mr Crowley said.

However, Fianna Fail said Mr Crowley had been given assurances about his staff by ALDE. ALDE itself says it cannot understand the move and said Mr Crowley is now “sitting with Eurosceptics, anti-Europeans, Unionists and Tories”.

The ECR group was established in 2009 after British Tory leader David Cameron, then in opposition, took the Conservatives out of the EPP because it was too federalist in its European outlook.

The group has voted since the election to admit Alternative für Deutschland, a German party that calls for weaker member states to leave the euro currency and opposes a free trade agreement with the US.