Former SDLP leader Mark Durkan is set to run as an election candidate for Fine Gael.
It comes just weeks after the SDLP announced a link-up with Fianna Fail.
The former Foyle MP is now set to run as a Fine Gael candidate in Dublin at the next European election in May.
Fine Gael senator Neale Richmond described Mr Durkan as a “serious addition to the Fine Gael family at this crucial time for our island”.
The 58-year-old led the SDLP from 2001 to 2010, taking on the mantle from Nobel Peace Prize winner John Hume.
He briefly served as Northern Ireland’s deputy first minister alongside the Ulster Unionist Party’s first minister David Trimble.
Mr Durkan stepped down as SDLP leader to concentrate on his job as Foyle MP.
His defeat by Sinn Fein’s Elisha McCallion at the 2017 general election was considered to be one of the surprises of that poll, which saw all three SDLP MPs lose their seats.
The Londonderry man has been a member of the SDLP since 1981.
An SDLP spokeswoman paid tribute to Mr Durkan.
“Mark Durkan provided the people of Derry with a first class public service for decades,” she said.
“He is a first class parliamentarian and his regrettable absence on the benches of Westminster has not gone unnoticed by parties and communities alike.
“We wish him well in the upcoming European election. Ireland can only benefit from having Mark Durkan back on a parliament bench.”
Fianna Fail’s branch in Londonderry said in a tweet it was “disappointed” by the move by a man of Mr Durkan’s calibre.
“Fianna Fail and the SDLP are the future on this island, Mark,” they added in the tweet.