In a move that took everyone by surprise, Mr Durkan has joined Taoiseach Leo Varadkar’s party just weeks after the SDLP announced a partnership with southern rival Fianna Fail.
The fallout from that decision saw a number of key figures resign from leadership positions within the SDLP, including South Belfast MLA Claire Hanna.
Now the party faces another high-profile departure with Mr Durkan’s decision to run for the European Parliament in Dublin.
The 58-year-old has been a member of the SDLP since 1981, taking over the party leadership from John Hume in 2001 before standing down in 2010.
He briefly served as Deputy First Minister alongside Ulster Unionist David Trimble.
The former MP for Foyle lost his Westminster seat to Sinn Fein’s Elisha McCallion in 2017.
Speaking in Dublin yesterday, Mr Durkan said he will suspend his SDLP membership in order to run as a Fine Gael candidate alongside former Tanaiste Frances Fitzgerald.
He added: “Obviously, going forward, I will be putting on a Fine Gael vest but I will not hide my SDLP jersey underneath.”
Mr Durkan said he wants to be part of a “strong team Ireland that stands for decency”.
“Given my own deep involvement in negotiating the Good Friday Agreement, I would really welcome the opportunity to contribute in that context in the European Parliament,” he said.
“The Taoiseach is following through on his statement last year that Northern Ireland citizens will not be left behind.
“This is a big ask for the people of Dublin, but as we see a huge disenfranchisement in Northern Ireland, the promises of rights that were framed in the Good Friday Agreement stand in deficit at the minute.
“In the broadest sense, I hope the people of Dublin look at this as an election of the greatest national importance.
“I hope this is a green jersey day with a strong turnout to send a message to Brexiteers that the Irish people as a whole value the EU in a fundamental way.”
Mr Durkan, who is married to Jackie and has one child Dearbhail, added: “I’m not going to pretend I’m not a Derry person, and I’m not going to pretend that my family and I won’t continue to live there.”
In a statement, the SDLP said: “Mark Durkan provided the people of Derry with a first-class public service for decades.
“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.”
However, Fianna Fail’s branch in Derry said in a tweet it was “extremely disappointed” at the move by a man of Mr Durkan’s calibre.
“Fianna Fail and the SDLP are the future on this island, Mark,” it said.
Sinn Fein MEP Martina Anderson was also critical of the decision.
She tweeted: “Not only has former leader Mark Durkan jumped ship from the SDLP, he is also walking away from any prospect of securing democratic European representation for northern citizens after Brexit.”
Mr Durkan follows in the footsteps of former SDLP member Austin Currie in running for Fine Gael. Mr Currie was a TD in Dublin West and presidential election candidate.
Mr Varadkar said his decision to invite Mr Durkan to run as a candidate was for the political future of the island.
“I don’t want the people of Northern Ireland to have no voice or representation in Europe, I don’t want them ever left behind again,” he added.