Regulations introduced by the UK Government to commission full abortion services in Northern Ireland are legally invalid, the Court of Appeal heard on Tuesday.
Counsel for a pro-life group also claimed subsequent amendments cannot rehabilitate legislation likened to a "corpse".
Earlier this year the High Court held that the Secretary of State was entitled to impose a deadline on the Stormont Executive to introduce a centralised system for abortions.
The Society for the Protection of Unborn Children (SPUC) is seeking to overturn that ruling amid claims that only elected representatives in Northern Ireland should decide on the issue.
The case represents the latest stage in a protracted legal battle over Northern Ireland's abortion laws.
In 2019 MPs passed legislation to decriminalise terminations at a time when devolution had collapsed.
But a centralised model to operate across Northern Ireland was not put in place due to an impasse at Stormont.
The liberalised regime change brought in by Westminster followed a report by the UN Committee on the Elimination of Discrimination Against Women (CEDAW), which found their rights were being breached by limited access to services.
Under the terms of the Northern Ireland (Executive Formation etc) Act 2019, former Secretary of State Brandon Lewis had to implement the CEDAW recommendations in Northern Ireland.
Section 9 of the Act imposed specific duties on him about the provision of abortion and post-abortion services.
Amid the continuing impasse, a direction was issued under the Abortion (Northern Ireland) Regulations 2021 for the Department of Health to set up full services by March this year.
Mr Lewis’s successor, Chris Heaton-Harris, has also pledged his commitment to the move.
In a challenge to the legality of the 2021 Regulations, SPUC claims that constitutional arrangements enshrined in the Good Friday Agreement meant the Secretary of State went beyond his legal authority.
The group is appealing a finding that section 9 of the 2019 Act gave “broad, expansive powers” as part of parliamentary sovereignty.
John Larkin KC questioned whether the legislation was designed to replace or secure the restoration of devolution.
“Did Parliament write a blank cheque?” he asked.
Counsel insisted that amendments introduced earlier this year illustrated the flaws in the 2021 regulations.
“Our argument is simply that the direction has no effect whatsoever. It doesn’t carry any normative weight,” he told the court.
Rejecting suggestions that the legislative changes rendered his point academic, Mr Larkin said: “The 2021 regulations are dead, and one can’t graft a healthy arm onto a corpse.
“Being invalid from the outset, they couldn’t then be cured afterwards.”
During submissions a comparison was drawn between other provisions within the Act in a bid to demonstrate constraints on the Secretary of State.
With that argument introduced for the first time, a barrister representing the Secretary of State requested more time to take instructions before continuing with the appeal.
Lady Chief Justice Dame Siobhan Keegan adjourned proceedings after stressing the importance of the case.
She said: “It is very regrettable that we do that, but we need to get this case fully and properly argued to allow us to decide on it.”
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