Fianna Fáil MEP Brian Crowley has left the liberal group in the European Parliament to join the political family currently hosting British Conservative MEPs, as well as a number of far right parties.
It is understood that Fianna Fáil leader Micheál Martin is "furious" at the move.
Fianna Fáil MEPs have been members of the European liberal party, the Alliance of Liberals and Democrats since 2009.
Today the Ireland South MEP, the only Fianna Fáil member to retain his seat in May's European Elections, formally announced that he is leaving the group to join the European Conservative and Reformist group.
In a statement, he said: "I have decided to leave the ALDE Group in the EU to join the ECR Group, which will allow me to provide a better service for my constituents.
"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."
According to sources, Mr Crowley had attempted to secure long-term contracts for two Irish staff who had worked for the ALDE group, but was told by the liberal group that it was not possible.
The staff had worked on the European Parliament economic affairs committee, and on the agriculture and environment committee.
Both had previously worked for the UEN, Fianna Fáil's previous group, as well as for ALDE between 2009 and this year.
Mr Crowley is understood to have lobbied Mr Martin to have the positions rolled over.
The Fianna Fáil leader is then thought to have contacted the group leader and former Belgian prime minister Guy Verhofstadt over the issue.
One staff member was to be retained, and the other given a temporary two-and-a-half year contract, which could be renewed.
It is understood Mr Crowley spoke to Mr Martin "four or five times" over the past week to have the positions renewed.
Mr Martin is understood to have told the Ireland South MEP that he did not have the authority as a Fianna Fáil MEP to move to the ECR.
Fianna Fáil has criticised Mr Crowley over today's move.
In a joint statement, party chairman Brendan Smith and chief whip Seán Ó Fearghaíl said: "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.
"Indeed, 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.
"ALDE had also committed to and agreed to Brian's request to keep on two administrative staff for the coming parliamentary session.
"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."
A spokesperson for ALDE has expressed surprise at Mr Crowley's decision.
She said that in conversations between Mr Martin and Mr Verhofstadt, Mr Martin made it clear that Fianna Fáil was still a member of the ALDE group, and that Fianna Fáil's membership of the group was not in question.
She said: "We can only assume this was a unilateral decision by Brian Crowley."
ALDE sources say the group was unable to agree to Mr Crowley's request for two group staff to be retained because the group had lost seats in the European elections and was therefore under financial pressure.
The ECR has been home to British Prime Minister David Cameron's Conservative Party ever since he pulled his MEPs out of the main centre right grouping, the European People's Party.
In recent weeks, the anti-immigrant Danish People's Party as well as the True Finns joined the ECR following the election.
It is understood Mr Crowley's decision is related to a long-standing and simmering resentment by Fianna Fáil MEPs at the decision by then-Taoiseach Brian Cowen to join ALDE in 2009.
Mr Crowley’s switch to the ECR will further strengthen the eurosceptic political party as it has been engaged in a struggle with ALDE to become the third biggest grouping in the European Parliament.
Mr Crowley's move means that the ECR now has 69 seats compared to ALDE's 66.
In recent weeks the ECR convinced the German eurosceptic party Alternative für Deutschland (AfD) that its six MEPs should join its ranks, infuriating Chancellor Angela Merkel who views the AfD as domestic opponents.
On that occasion, Mr Cameron attempted to block the admission of the AfD but Tory MEPs were powerless to prevent it.
The ECR was founded in 2009 after Mr Cameron pulled his Tory MEPs out of the EPP in a bid to appease eurosceptic Conservative backbenchers.
In the outgoing European Parliament Tory MEPs were joined by eurosceptic MEPs from the Czech Republic and Poland.
Following the elections on 25 May, the Flemish nationalist party, the N-VA, also joined the ECR.
When Fianna Fáil joined ALDE in 2009 it collapsed the group it was a member of, the UEN.
It is understood Fianna Fáil MEPs were always unhappy at the manner of the move, which was announced by former taoiseach and party leader Brian Cowen, apparently without consultation.
There have been tensions between Fianna Fáil MEPs and mainstream liberal MEPs from other member states over social and moral issues, but also on issues related to farming and fisheries.