The former Progressive Democrat Senator, Ms Helen Keogh, has said she will not relinquish her Seanad seat following her defection to Fine Gael.
Ms Keogh, who dealt a body-blow to the PDs last night by announcing she was joining Fine Gael, angered her old party by insisting she would use her Seanad vote to oppose Government policy.
The Progressive Democrat leader, Ms Harney, said that the honourable thing for Ms Keogh to do would be to give up her Seanad seat as it was a Taoiseach's appointment.
A spokesman for Fine Gael said last night there was no precedent for such a move. He said that in 1994, when Mr Albert Reynolds stepped down as Taoiseach, his seven Seanad nominees remained in the John Bruton government.
The departure of Ms Keogh had been signalled in the last week. It came hours after a meeting with the Progressive Democrat parliamentary party when she was told by her colleagues to make a decision either way.
Senator Keogh's defection to Fine Gael reduces to seven the number in the Progressive Democrat Parliamentary Party - four TDs and three Senators.
Ms Keogh will now contest the next by-election for Fine Gael in Dun Laoghaire where the party's two sitting TDs, Ms Monica Barnes and Mr Sean Barrett, are retiring from politics. Ms Keogh held a Dail seat for the PDs in Dun Laoghaire from 1992 to 1997.
She said last night she has grown increasingly "uneasy" in recent times at the way in which the Progressive Democrats were "melting back into Fianna Fail".
"As a founder member of the Progressive Democrats 15 years ago, I had great hopes that the party could forge a dramatic new direction for Irish politics. Some of this has been achieved, but my overall feeling is one of disappointment."
Ms Keogh said that in the current Government, the Progressive Democrats had "failed to sustain an ongoing political relevance" and that the ideals which first attracted her into active politics with the PDs were not being addressed.
"The party now lacks the hunger and the energy to fight for its relevance in this very unbalanced Government where the domineering Fianna Fail party has killed off any chance that the Progressive Democrats had to make an impact."
Ms Keogh said the recent Government decision to appoint Mr Hugh O'Flaherty to the European Investment Bank, coupled with the Tanaiste's subsequent decision to accept this, brought her to her decision to quit the party.
It is also known that Senator Keogh was upset at attempts by the party leadership to impose PD Councillor Ms Fiona O'Malley, a daughter of the former PD leader Mr Des O'Malley, as a candidate in the Dun Laoghaire constituency in the next general election.
Ms Harney said she regretted Senator Keogh's departure but denied that it was another nail in the coffin of the PDs.