The Secretary of State for Northern Ireland exceeded his powers by issuing directives to radically expand abortion provision in the province, a pro-life group has claimed.
The Society for the Protection of Unborn Children (SPUC) made the argument at the High Court in Belfast, where it is seeking a judicial review into powers given to Brandon Lewis earlier this year to direct Stormont on bringing in full abortion services.
Laws passed by Westminster when Stormont was suspended radically alter abortion provision in Northern Ireland, allowing for the first time abortion up to birth on the grounds of disability.
Lewis used his new powers in July by directing Northern Ireland's First and Deputy Ministers and the health department to implement the abortion laws.
They have been given until March 2022 to ensure the services are in place.
Representing SPUC at the two-day hearing this week, John Larkin QC argued that the powers effectively turn Lewis into a "super legislator" capable of side-stepping Stormont and the devolved institutions.
The powers, he said, allow Lewis to "make whatever laws he wished and transform the Northern Ireland constitution in ways that the Assembly could not".
"That cannot be right," said Mr Larkin.
A judgment is not expected for several weeks.
Liam Gibson, SPUC's Policy and Legal Officer, called it "a gross abuse of political power" that is incompatible with the terms of the Good Friday Agreement.
"We believe that we've shown the court that the Secretary of State has acted unlawfully by ignoring the legislative process and attempting to impose his own will on the people of Northern Ireland," he said.
"There is a real danger that if Brandon Lewis is allowed to get away with this power grab then it will permanently damage the credibility of accountable government in Northern Ireland.
"The principles of self-government are central to the Belfast Agreement. If London decides to abandon those principles then it has abandoned the Good Friday Agreement."
The court also heard a legal challenge from the mother of a 15-year-old with Down's syndrome.
Rosaleen McElhinney says it is "discriminatory" to allow abortion up to birth for disabled babies while limiting it to 24 weeks for non-disabled babies.
Legal papers submitted to the Belfast High Court by Ms McElhinney's lawyers state that the new abortion regime "operates to perpetuate stereotypes towards persons with disabilities".
A similar legal challenge against disability discrimination has been brought to England's High Court by campaigner Heidi Crowter, who has Down's syndrome. She lost her case but plans to appeal to the Supreme Court.
Nola Leach, CEO of CARE, which is supporting Ms McElhinney, said: "In allowing abortion up to birth for disabled babies – far beyond the 24-week limit for other preborn babies – it implies that disabled lives have less value.
"This regressive and discriminatory policy should have no place in Northern Ireland or any other part of the UK.
"Like other NI citizens, Rosaleen is also alarmed by diktats from UK government ministers seeking to bypass the NI devolution settlement. Abortion was imposed on Northern Ireland from afar when its government was in crisis.
"This contempt for devolution would never be shown towards the governments of Scotland or Wales. Legislators in other parts of the UK should be concerned by the precedent set by this kind of intervention."