MEP Nessa Childers has decided to contest the Dublin constituency in the upcoming European elections in what will come as a shock to Labour.
Ms Childers dramatically quit the party in April before claiming Labour was part of a government that was "actually hurting people".
The decision came as a major blow to the Labour leadership, given that Ms Childers was the party's only elected MEP.
It has now emerged that she is switching from the Ireland East constituency and will now run in Dublin. Ms Childers has told supporters she now felt better placed to win one of the three seats in the capital as a result of a series of recent boundary changes.
She is due to announce she will run in Dublin on Wednesday. But the decision to contest the three-seat Dublin constituency will put Ms Childers head-to-head with her former party colleague, Emer Costello. Ms Costello, who was co-opted on to the European Parliament in 2011, is the only Labour candidate running in Dublin.
Speaking to the Irish Independent, the former Dublin Lord Mayor said she was not concerned about Ms Childers's decision.
"I think independents lack a real function in the European Parliament. They don't belong to a political group so they don't have much influence. But Nessa can run where she pleases, that's up to her," she added.
Senior Labour figures are extremely concerned about the prospect of losing both of their sitting MEPs. Ireland South MEP Phil Prendergast is facing a challenge from within her own party during a selection convention in coming weeks.
Labour's Seanad finance spokesman, John Gilroy, has said he would challenge Ms Prendergast for her place on the ticket. If he succeeds during the selection convention, the party will have to decide whether to add Ms Prendergast on to the ticket.
Otherwise, it is likely she will run as an independent in May's elections.