Anti-abortion campaigners claim they have "no alternative" but to mount a legal challenge if the Scottish government approves plans for women to take abortion medication at home.
The Society of the Protection of the Unborn Child (SPUC), said it had already sought legal advice.
It said it had been assured "a good chance of success".
The Scottish government said it had worked hard to ensure women could always access clinically-safe services.
In October the chief medical officer told health boards the drug misoprostol could be taken outside a clinical setting.
Dr Catherine Calderwood said it was "significant progress" that women in Scotland who are up to nine weeks pregnant could take the second dose of the drug at home if they wanted, saying this would allow them "more privacy, more dignity".
Campaign groups including Engender, Amnesty Scotland and Rape Crisis Scotland welcomed the move, while Prof Lesley Regan, president of the Royal College of Obstetricians and Gynaecologists (RCOG) described it as "admirable".
But the SPUC's John Deighan claimed that the potential health risks for women were "horrific".
He said: "There would be no medical oversight and this development will result in dreadful threats to women's health."
SPUC said it had received detailed legal advice from an advocate who specialises in human rights cases, who said that under the law the medication could not be taken without some form of medical supervision.
The advocate stated: "In my view, the taking of the abortifacient drugs must be done under the supervision either of a registered medical practitioner, or by some other suitable member of staff who is acting under the control of a medical practitioner.
"It cannot be done by a patient unsupervised, at home or elsewhere."
He added: "As the approval anticipates the patient administering the drug to herself without medical supervision, that approval proceeds upon a misdirection as to the requirements of the 1967 Act, and is accordingly unlawful."
Mr Deighan added: "Our advice is clear and we really have no alternative but to challenge these proposals which go to the core of our beliefs in the right to life for unborn children and the health and wellbeing of their mothers."
The government plans are not a change to abortion law but to powers available within the Abortion Act 1967.
A Scottish government spokeswoman said: "We've worked hard to ensure women are always able to access clinically safe services.
"Scotland is the only part of the UK to offer women the opportunity to take misoprostol at home, when this is clinically appropriate, a decision that allows women to be in control of their treatment and as comfortable as possible during this procedure."