The first Progressive Democrats councillor to quit the party since the general election will today announce that she is to join Fine Gael.
Dún Laoghaire-Rathdown councillor Mary Mitchell-O'Connor will be unveiled by Fine Gael at a photocall with party leader Enda Kenny today.
Her departure will raise new concerns within the Progressive Democrats that more councillors will defect prior to the 2009 local elections.
In addition, it will re-ignite questions as to the future intentions of one of the party's two remaining TDs, Galway West's Noel Grealish.
In a brief statement announcing the defection last night, Fine Gael chose not to identify Cllr Mitchell-O'Connor by name.
Cllr Mitchell-O'Connor, who intends to hold on to the seat she won on Dún Laoghaire-Rathdown council at her first attempt in 2004, has notified the PDs about her decision to quit.
Last night, the PDs said they believed Cllr Mitchell-O'Connor had been offered a Dáil nomination to defect. "We are disappointed that she is leaving and we wish her well."
However, Fine Gael sharply denied that any inducements had been offered.
Fine Gael won one seat in Dún Laoghaire in May with the return of Seán Barrett, though one of its defeated candidates, Eugene Regan, has been elected to the Seanad.