Legal action is being planned to challenge whether new Government legislation which led to the return of Stormont restores the “core constitutional provisions” of the Acts of Union.
Jamie Bryson, from the Unionist Voice Policy Studies group, has also asked new agriculture minister Andrew Muir to confirm that all checks on goods destined for Northern Ireland from the rest of the UK will end when the legislation clears all parliamentary stages.
The DUP gave the green light for the recall of the Stormont Assembly on the back of its deal with the Government following its two-year boycott.
Last week, two pieces of legislation contained in the agreement to resurrect devolution were fast-tracked through the House of Commons.
DUP leader Sir Jeffrey Donaldson said the new arrangements have effectively removed the so-called Irish Sea border for goods moving from Great Britain to, and staying in, Northern Ireland.
The UK Government has said the changes cut post-Brexit bureaucracy on Irish Sea trade and ensure EU law alignment will no longer automatically apply in Northern Ireland.
The motions will be considered by the House of Lords on February 13 and peers must approve them before they can become law.
But Mr Bryson said his group will “test the DUP’s claims” in court if necessary.
The loyalist activist has been a vocal critic of the deal to restore Stormont, appearing at a public protest meeting in Co Tyrone last week.
He said: “The DUP have made many bold claims, but the central being they have removed the Irish Sea border by removing all checks and paperwork for goods destined for Northern Ireland.
“Therefore, we have written to Daera (Department of Agriculture, Environment and Rural Affairs) to firstly ask for confirmation that all checks have now stopped on goods using the UK Internal Market Scheme, and put them on notice of our intention to commence legal proceedings once the new constitutional regulations have passed the House of Lords in nine days’ time.”
Mr Bryson said the proposed legal challenge would be to test if the new arrangements amount to the restoration of the “core constitutional provision” of the 1800 Acts of Union.
He added: “That means no EU law, no checks, no paperwork.
“So, we go to test the DUP’s claims.
“And if, via this proposed legal action, it becomes clear that in fact the Acts of Union aren’t restored, and contrary to the clear and emphatic claim of Sir Jeffrey that all checks have been removed, that they aren’t actually stopped, then that is going to be very interesting indeed.”
Previous court proceedings have ruled that, while Article 6 of the the Acts of Union was “modified” by the post-Brexit Northern Ireland Protocol, that was done with the express will of a sovereign parliament and so therefore was lawful.
A spokesperson for the Department of Agriculture said: “The department can confirm it has received correspondence from Mr Bryson and will respond in due course.”