Fidelma Healy Eames has become the first Fine Gael senator to definitively declare she will vote against the Government's proposals to legislate on abortion unless the suicide clause is removed.
Ms Healy Eames contacted her party leader Enda Kenny and general secretary Tom Curran yesterday to inform them of her final position.
The decision of Ms Healy Eames, which is believed to have received a chilly response from the FG head office, means the Government will no longer have a majority in the Seanad once Ms Eames votes against the bill and loses the whip. It is also likely to increase the pressures on other senators such as Paul Bradford and Michael Mullins to consider their own position.
Speaking to the Sunday Independent, Ms Healy Eames said: "I would have voted for the bill had one shred of evidence been produced to say that women's lives would be saved. It hasn't.
"It was the hardest decision I have ever made," she added. "I am deeply disappointed that I find myself having to vote against my party, Fine Gael, and colleagues I respect on this bill. It is a lonely place, but I feel very free."
She also warned "it is an unjust and dangerous bill permitting abortion on a threat of suicide without any sound psychiatric basis''.
Ms Healy Eames warned that the absence of specified time limits on abortion ran the danger of creating "a culture change in our public hospitals where one baby is being saved and one is being aborted at the same gestational age''.
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It is disappointing, she said, that "the Government at the very least did not introduce time limits which are the norm even in countries with very permissive abortion regimes''.
Mr Kenny subsequently texted Healy Eames to offer to meet, but said he wouldn't change the legislation.
Meanwhile, Fine Gael TD Regina Doherty revealed in dramatic detail the intimidation faced by TDs who support government proposals to legislate on abortion.
In an interview on Newstalk's Sean Moncrieff show, Ms Doherty said these included "physically threatening public confrontations, spitting, cyber stalking from locations as distant as internet cafes in Zagreb and threats to slit TDs from 'your throat to your navel and watch your entrails spill out'".
Ms Doherty believes that she has been singled out for particular vitriol because of her decision, despite the TD's strongly held conservative views, to support the Government's X Case legislation.
In a week where the independent TD John Halligan told the Dail that one female TD had been warned her throat would be cut, Ms Doherty also indicated her belief that certain women were being singled out for "targeting'' by extremists.
Ms Doherty provided personal details of one dramatic encounter where she had raced out to an estate in her constituency to confront a group of protesters.
She said that in taking on "the two chaps that were delivering the leaflets'' she was "foolish, probably, now looking back on it''.
She said one protester was "practically spitting at me, threatening me, pointing his finger at me and sticking his face into my face''.
She also claimed "some people are genuinely, a bit unbalanced'' citing the example of "one individual called 'Whispering Ghost' who is sending all the Fine Gael TDs "at the moment, anonymous emails... and his IP address seems to be coming from somewhere in Zagreb''.
Ms Doherty added: "He's the gentleman behind the 'slitting of the throat to your navel' email. We received another choice one from him at the back end of last week and he's made reference to what he's going to do to certain members of our party''.
Ms Doherty also noted the aggression she had experienced. "I'm not talking about normal respectful differences of opinion and people genuinely trying to get me to change my mind back," she said.
John Halligan the independent TD, in the Dail last week slammed the refusal of a "blood-stained'' Catholic church to condemn the tactics of anti-abortion protesters.