Prime Minister Boris Johnson will not want the legal action challenging the Northern Ireland Protocol to gain "any traction" - or further supporters in England - as it could weaken his negotiating stance, a legal expert has said.
The DUP's Arlene Foster, Nigel Dodds, Sir Jeffrey Donaldson and Sammy Wilson, as well as the UUP, are joining the legal challenge begun by Belfast-born Baroness Kate Hoey, TUV leader Jim Allister and former Brexit Party MEP Ben Habib.
Unless alternative post-Brexit trade arrangements are put in place which secure their consent, they will launch judicial review proceedings in Belfast.
Former Northern Ireland Attorney General John Larkin QC is providing legal representation for the group.
Mrs Foster, the First Minister, said the protocol had "driven a coach and horses through both the Act of Union and the Belfast Agreement".
As part of the Brexit Withdrawal Agreement, the protocol sees checks on goods between Great Britain and Northern Ireland, creating a de facto Irish Sea border - angering unionists.
Speaking to the Belfast Telegraph, Dr Austen Morgan, a barrister in London and Belfast, said a "possible weakness" in the legal challenge was if a judge ruled that problems resulting from the protocol could be resolvable under the agreement.
Mr Morgan was involved in the building of a judicial review in London against the Irish backstop agreement between the EU and former Prime Minister Theresa May.
Although that legal challenge did not go ahead as the backstop was eventually scrapped, Mr Morgan said he did see a lot of similarities in the two cases but warned against predicting success "easily" as it will be a "long, rocky road".
"If Europe was to think cleverly, which I'm not assuming they will be, they would get going on the joint committee to try and resolve these problems and knock them on the head one by one," he explained. "That would be the clever thing to do and that would be a way of depriving the claimants of any case in court.
"In fact, the weakness in their case might well be that a judge would say this agreement is not an agreement imposing things on you. This agreement contains lots of provisions about working together to resolve problems, therefore you haven't got a problem if it's potentially resolvable under the agreement."
Mr Morgan added that the possible "chink in Boris's armour" is the anti-Europe "noise" coming from Northern Ireland, which could inspire English Brexiteers to back unionist colleagues here.
Looking at the politicians who have signed up to the judicial review, Mr Morgan said it showed a "huge withdrawal of consent from the government by unionists".
"There's a lot of complaining about English soil attached to English plants and there's a lot of complaining about stuff not being sent from GB to Northern Ireland," he continued.
"There must be substance to the complaints. Now why Brussels is happy to tolerate that I do not know, other than Michel Barnier has taken his eye off the ball, having had his eye on the ball year after year when he was negotiating."