BREXIT laws impose no obligation on the United Kingdom to negotiate a withdrawal agreement with the European Union, the High Court in Belfast heard yesterday.
Counsel for Prime Minister Boris Johnson insisted that once notification was given, the departure process became "inexorable".
Responding in legal challenges to leaving without a deal, Tony McGleenan QC contended the legislation does not impose a duty on a member state to agree terms.
Mr McGleenan said: "Article 50 anticipated precisely the concept of a no-deal exit from the EU."
His analysis came as legal attempts were stepped up have the Prime Minister compelled to ask for an extension beyond the current October 31 deadline for quitting Europe.
It was claimed that Mr Johnson may "sabotage" a new law to block a no-deal Brexit.
Lawyers involved in three separate cases being dealt with in Northern Ireland pressed for either a judicial order or undertakings that he will abide by the requirement to seek a delay.
Barry Macdonald QC, representing one member of the public granted anonymity, told the court the application was in response to fast-moving developments, and indications from Downing Street of an unwillingness to comply.
One of the cases has been brought by victims campaigner Raymond McCord.
The Belfast man, whose son Raymond Jr was murdered by loyalist paramilitaries in 1997, alleges that a no-deal Brexit will plunge Northern Ireland into chaos, economic misery and threaten the peace process.
According to his legal team the government are unlawfully trying to quit the EU without an agreement at any costs.
They claim that such an exit departure breaches the European Union (Withdrawal) Act 2018 which safeguards the 1998 Belfast Agreement.
But rejecting that assessment, Mr McGleenan argued that the courts should not even be dealing with issues of foreign affairs and international relations.
He submitted that the legal mechanism by which a member state can leave the EU does not even impose a requirement to give a reason.
"Once notification is provided, the procedure set out in Article 50 takes an inexorable course," the barrister said.
"The obligation in the Article is on the Union to negotiate, there's no obligation on the member state to negotiate.
"Article 50 in its entirety anticipates the possibility of withdrawal without an agreement."
According to counsel the legislative mechanism was designed to ensure member states could not be permanently kept in the EU if they have given notification of an intention to exit but been unable to reach a deal.
Lord Justice McCloskey was also told the Withdrawal Act does not contravene legislation specific to Northern Ireland.
"The regulations which modify exit day cannot conceivably be considered to be contrary to the terms of the Northern Ireland Act," Mr McGleenan added.
Part of the case centres on an alleged failure to take into account Operation Yellowhammer - the codename given to the government's plans for dealing with any consequences from a possible no-deal Brexit.
The mitigation scheme for coping with potential disruption came to light after confidential documents were published in the press.
A new affidavit, authorised by Chancellor of the Duchy of Lancaster Michel Gove, was introduced in court in an an attempt to rebut the allegations.
Combined with a statement Mr Gove made to the Commons earlier this month explaining the plan was for a worst-case scenario, Mr McGleenan insisted that the ground of challenge must fail.
"Operation Yellowhammer isn't a document, it's an ongoing programme to assess risk which is dynamic in form," he said.
"It's apparent the government has considered carefully, assessed and made provisions for the contingency that may arise from exiting without a withdrawal agreement.
"That aspect of the challenge is also non-sustainable."
Earlier, Northern Ireland's Attorney General told the court that domestic UK law cannot prevent a no-deal, because such a departure is enshrined as a possibility in Article 50 of the Lisbon Treaty.
John Larkin QC also contended that legislative arrangements for the peace process were not reliant on the UK staying in the EU.
The British-Irish Agreement which underpins the Good Friday Agreement refers to Britain and Ireland as "friendly neighbours and as partners in the European Union".
The Attorney said: "That can't be boosted or given a steroid injection as to make it an obligation of continuing membership of the EU on one or both parties."
