The government must announce later on Monday whether to allow the Stormont brake to be pulled after a request by unionist parties in the Northern Ireland Assembly.
The brake is part of Northern Ireland’s Brexit deal and gives the assembly the power to object to changes to EU rules that apply in Northern Ireland.
A Democratic Unionist Party (DUP) motion to pull the brake, in a bid to stop new EU rules on packaging and labelling of chemicals from coming in, was backed by all eligible unionist assembly members in December.
It is up to the government to decide whether the brake can be formally triggered.
The brake formed part of an overall package of measures – known as the Windsor Framework – agreed by the UK and EU two years ago, aimed at resolving problems with post-Brexit trade arrangements in Northern Ireland.
The framework is the special Brexit deal which applies to Northern Ireland and means it continues to follow some EU laws relating to goods.
The brake was designed to give Stormont’s politicians a greater say before any amended EU rules could take effect in Northern Ireland.
Entirely new rules are dealt with under a similar process known as an applicability motion.
The move by unionist parties before Christmas is being viewed as a first test of the mechanism since power-sharing returned at Stormont almost 12 months ago.
There are restrictions on its use for assembly members who wish to use the brake.
Firstly, the measure can only be used in the “most exceptional circumstances and as a matter of last resort”.
The mechanism also cannot be used for “trivial” reasons and those seeking it to be pulled must demonstrate that the rule being challenged is having a “significant” impact on everyday life in Northern Ireland.
Unionist parties had a two-month “scrutiny period” from the publication of the EU law to make their request and for the government to respond, making Monday the deadline for a decision by Northern Ireland Secretary Hilary Benn.
What are the government’s options?
If it decides that the threshold has not been met, Benn will write to the Northern Ireland Assembly Speaker outlining why, and the amended EU law will apply in Northern Ireland.
If the government has determined that the conditions have been met, it will formally notify the EU and the new or changed law will not apply.
There would then be “intensive consultation” between the UK and EU on the matter.
The Joint Committee – the UK-EU body responsible for overseeing the Brexit deal – would be required to discuss the rule in question.
Once those discussions are concluded the UK government can either send it back to the assembly for a cross-community vote or decide the rule should not apply in Northern Ireland.
At that stage, the government could still avoid a Stormont vote if it assessed there were “exceptional circumstances” including an assessment that the rule would not create a new regulatory border between Great Britain and Northern Ireland.
If the UK decided not to adopt the rule, the EU could take “appropriate remedial measures”, which could include measures to address the fact that NI goods may no long fully comply with EU law.
What have Stormont parties said?
When his party tabled the motion in December in a bid to see the brake pulled, DUP leader Gavin Robinson said the new EU regulation would have a “significant and prolonged” impact in Northern Ireland if it took effect.
He said it would introduce a host of new requirements for labels attached to chemical products, including new minimum font sizes and rules around spacing, making current labels unusable for the majority of products.
“Critically, these changes would not be required for products on the market in Great Britain,” he added.
First Minister Michelle O’Neill, of Sinn Féin, said other parties had not been informed of the DUP’s actions and warned it could potentially cause instability for businesses in Northern Ireland.
The leader of the official opposition at Stormont, Matthew O’Toole, described the use of the brake as “a stunt”.
“We warned that the so-called Stormont brake would be used for crude partisan purposes and so it has proven,” the Social Democratic and Labour Party assembly member said.
Ahead of the decision, Traditional Unionist Voice leader Jim Allister said that his party had “no illusions” about the limitations of the brake.
“The main impact of this experience must be to demonstrate to ourselves that no self-respecting people can willingly submit to this kind of injustice as a long-term arrangement,” he added.
“Rather than working with this injustice, accommodating it, going forward unionism needs to stand united against it.”
Analysis: NI secretary facing big decision
By Enda McClafferty, News NI political editor
You could call it a make or brake moment for Hilary Benn.
If he agrees with unionists and pulls the Stormont brake to halt EU changes to chemical packaging, he risks damaging relations with Brussels.
This is coming a week after the government appointed a senior civil servant to turbo charge the new EU relationship reset.
But if Benn decides the threshold to pull the brake has not been met, he will infuriate some unionists and energise others who regard the Stormont brake as nothing more than a gimmick.
Such a scenario in the past might have triggered a crisis at the heart of Stormont but not any more as there is no appetite to collapse the institutions.
But, whatever the outcome, expect the Northern Ireland secretary to talk up the prospect of a new deal with the EU removing the need for any brake.