The architect of the Good Friday Agreement is to take the British government to court over the proposed Brexit backstop arrangement, claiming it breaches the peace deal.
Lord Trimble announced he planned to initiate judicial proceedings as Theresa May confirmed she will visit Northern Ireland on Tuesday in a bid to calm fears about a hard border being put in place after Brexit.
The prime minister will give her “absolute commitment” to avoiding one, despite failing to get a deal with the EU to prevent such a scenario through parliament.
She will also meet local businesses on the trip that foreshadows her return to Brussels.
Lord Trimble’s announcement will come as a blow to those efforts.
The proceedings seek to remove the Northern Ireland protocol, which includes the controversial backstop.
The backstop is the insurance policy which kicks in if the UK and EU fail to strike a trade deal to avoid a physical border being created between Northern Ireland and the Republic.
But Lord Trimble, who was First Minister of Northern Ireland from 1998 to 2002, is proposing an interim free trade agreement in goods and agriculture, which would eventually become part of the comprehensive agreement between the EU and the UK.
He received a Nobel Peace Prize in 1998, jointly with John Hume, for his role in bringing an end to the Troubles.
Leader of the Democratic Unionists Arlene Foster confirmed her party would meet with Mrs May.
She said: “Our message to the Prime Minister has been consistent. The backstop is the problem. It drives a coach and horses through the Belfast Agreement’s principle of consent. If implemented, it would build a new border between Northern Ireland and the rest of the United Kingdom. Such an outcome would undermine both the economic and constitutional integrity of the United Kingdom.
“Parliament has spoken. A majority has rejected the current backstop. The European Union must now accept the need for the withdrawal agreement to be re-opened. The toxic backstop must be dealt with.
“This is not a time for intransigence. It is time to respect unionists and nationalists alike in Northern Ireland and deliver a deal which is sensible and practical.”
She said border communities should not have “fears about tall tales” amid rumours of barbed wire and checkpoints.
Mrs May is expected back in the Belgian capital to push EU leaders to re-open negotiations on the divorce deal she struck in November.
Downing Street has set up a taskforce to examine one plan dubbed the Malthouse Compromise to change the backstop.
Brexit Secretary Stephen Barclay is holding talks with senior Leave and Remain Conservative MPs to thrash out the Malthouse Compromise.
It has been given crucial support for development by civil servants, with those from HMRC, the Cabinet Office Europe Unit and Number 10 drafted in.
Mrs May said she was engaging with it “sincerely and positively”, with more meetings planned to scrutinise it on Tuesday and Wednesday.
But senior Labour MP Hilary Benn dismissed the idea as he led a group from the Commons Brexit select committee on a trip to Brussels.
“Personally, I don’t see how it can work, particularly in the very short amount of time that there is left,” he said.
The EU has also ruled out changing the deal that took over 18 months to broker.
“The backstop is part of the withdrawal agreement, and the withdrawal agreement is not open for re-negotiation,” said a spokesperson for Donald Tusk, the EU Council president.