A “CRITICAL moment” that puts new pressure on the Union and Boris Johnson’s leadership – challenges to the Northern Ireland Protocol have profound implications for the UK, experts say.

Just two months after the transition period ended and the UK finally, as the Leave motto goes, “took back control” of its borders and trade, one of the linchpins of Boris Johnson’s Brexit deal is the subject of both political and legal challenge.

And as tensions heighten over the operation of new trade rules, experts say there’s a growing sense of “betrayal” by the Conservatives and uncertainty over Northern Ireland’s place in the Union.

Last weekend a judicial review challenging the protocol – drawn up to avoid a hardening of the land border with the Republic of Ireland by allowing Northern Ireland to remain in the EU single market for goods – was launched by an alliance of unionists including former Labour MP Kate Hoey, former MEP Ben Habib and Jim Allister, leader of the Traditional Unionist Party.

The Ulster Unionist Party and Arlene Foster’s Democratic Unionist Party (DUP) – once key Brexit allies of the Tories under Theresa May – have joined them and the latter has a five-point plan over what it calls a “growing crisis” over the border now effective in the Irish Sea under the terms negotiated by leaders in London and Brussels.

In a joint statement with Maros Sefcovic of the European Commission on Wednesday, Cabinet Office Minister Michael Gove said both sides are committed to the “proper implementation” of the protocol, with the PM’s spokesperson insisting “discussions are ongoing” about “outstanding problems”.

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By the next day, Euro-sceptic Tory MPs in the European Research Group were urging Johnson to scrap it altogether.

And on Friday, Northern Irish Agriculture Minister Gordon Lyons of the DUP stopped construction on inspection facilities for agri-food goods arriving from Great Britain in a move branded a “stunt” by Sinn Fein’s Michelle O’Neill, the deputy first minister at Stormont.

Lyons said he was acting over the “practical difficulties” caused by the protocol, claiming the Internal Market Bill brought by Johnson’s Government over inter-UK trade gave him the “legal duty” to act.

Officials, he said, warned that port control posts are unlikely to cope when the grace period exempting retail agri-food goods from EU export health certification ends on March 31, with supply chains into Northern Ireland also unlikely to hold up when the regulatory red tape increases in April.

It’s a situation under intense scrutiny on the island of Ireland, and one political experts say is a direct result of the last-minute dealing that saw terms reached only in days from December 31, when the transition period expired.

“The current version of the protocol was negotiated in a matter of months,” says Jess Sargeant of the Institute for Government.

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“Most of the time preceding that was focused around Theresa May’s plan for the backstop, which would have looked quite different. It’s not surprising that some of the problems that weren’t thought of throughout the negotiations are now becoming apparent.”

“We didn’t know what we were transitioning into,” says Professor Katy Hayward of Queen’s University Belfast. “What Northern Ireland needed was for the rest of the UK to be aware of its distinct position.

“Because we are talking about borders, of course people feel immediately anxious about that because that goes to the very heart of the political division in Northern Ireland.

“It feels very much like a critical moment and a real test for the flexibility and relationship between the UK and the EU. Unionists do feel that this is an absolutely critical matter to engage upon. They are throwing everything they can at this.”

Dr Mary Murphy of University College Cork says thanks to its “complex and cumbersome and quite convoluted” rules, “this formulation of Brexit was always going to be disruptive”.

“It has definitely pushed the united Ireland argument,” she goes on. “We are talking about a united Ireland in a way that just wasn’t the case a few short years ago.

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“People generally across the island are talking about it. It’s not a serious, deep, detailed discussion, but there’s a sense that a referendum may well be on the horizon in a not too distant future. We are also seeing academics start to look at this in a much more serious way.”

Recent opinion polling in Northern Ireland saw Johnson and his Northern Ireland Secretary Brandon Lewis both rated “bad/awful” by 73% of those asked. Unionists, Hayward says, think Johnson “has betrayed them”.

The UK Government, she states, “are more explicitly pro-Union than their predecessors would have been but at the same time they are completely not trusted by unionists and are seen to have damaged the Union by their decisions regarding Brexit.

“It’s a relationship of mutual suspicion.”

It's a situation that’s opening up old rivalries and factions in the Tories, according to SNP Northern Ireland spokesperson Richard Thomson, with “ambitious people circling around” the PM.

“Brexit is like pass the parcel,” he says. “Only nobody in the Conservatives wanted to be holding the parcel when it was opened. I don’t think anybody is quite ready to start the music once again.

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“There’s a direct impact on Scotland. We have an Irish Sea trade barrier where there was none before. There is a general lack of respect for devolution.

“It’s brought the border issue back into consideration in Northern Ireland in a way it hasn’t been since the Good Friday Agreement. There is a substantial tranche of opinion wondering whether or not there are better alternatives than have been offered.”