A no-deal Brexit would cause chaos in Northern Ireland, a legal challenge in Belfast heard.
Prime Minister Boris Johnson is trying to act oppressively and contrary to the constitutional position of the country, counsel for Troubles victims campaigner Raymond McCord alleged.
The anti-Brexit activist claims Prime Minister Boris Johnson's threat to leave the EU on 31 October, with or without an agreement, is unlawful.
His barrister, Ronan Lavery QC, praised the EU for its contribution to peace as Northern Ireland tried to become less tribal.
He said: "To leave without a withdrawal agreement would create chaos and economic misery and a real threat to the peace process in Northern Ireland - it would be madness."
Mr McCord's legal team began a judicial review at Northern Ireland's High Court on Friday.
His son, Raymond McCord Jr, was murdered by loyalists from the Ulster Volunteer Force in North Belfast in 1997.
He has repeatedly voiced concern about the impact of Brexit, which was rejected by a majority in the region, on the peace process.
All-island co-operation deepened following the 1998 Good Friday Agreement, which largely ended the 30-year conflict.
Campaigners are opposed to the creation of a hard Irish border after Brexit, which senior police officers believe could provide an opportunity for dissident republicans.
Mr Lavery said Northern Ireland was trying to move forward on a cohesive and non-tribal basis and was a model for what could be achieved when people put aside their differences.
He added: "The EU is a peace project."
He said the EU withdrawal process should serve the interests of the people of Northern Ireland.
"Generous means generous to the people of Northern Ireland; it cannot mean generous to the pursuit of English nationalism, it can only be something which serves the people of Northern Ireland."
He claimed there was no specific provision for leaving without a deal.
The barrister said the next step after the rejection of the Withdrawal Agreement by Parliament was for a statement to be made by the Government about how it proposed to proceed and making the policy clear.
Instead, he said, the policy was having to be divined from statements to the media as he scrutinised the legislation surrounding Brexit.
"Our very simple proposition is that, when you take this piece of legislation, nothing sanctioned or authorised the executive (government) to take the drastic step of leaving the EU without a deal.
"That needs to be expressly sanctioned by Parliament and it has not been.< "Any withdrawal is premised upon a deal."
He said the Prime Minister was seeking to act in a way which was "oppressive" to the people of Northern Ireland and contrary to the constitutional position.
Mr Lavery added: "I am not asking the court to do anything radical, I am simply asking the court for the law to be applied."
He said it was not possible for the UK to leave the EU outside the terms of legislation.
Lawyers whose clients oppose no-deal joined forces during Friday's hearing, claiming the UK's Brexit negotiations were a "sham".
They said the Government needed statutory underpinning from law for its powers.
Judge Mr Justice Bernard McCloskey observed that the Council of Europe, comprising leaders of the EU states, could not be compelled to do anything.
The case is due to continue on Monday when a barrister for the Government will outline his case.