• U.K. Prime Minister Rishi Sunak received a boost on Monday, as opposition leader Keir Starmer confirmed that his Labour Party would vote in favor of the deal, known as the Windsor Framework.
• The sticking point could come from Stormont, near Belfast, where the devolved Northern Ireland Assembly has been suspended for a year after the pro-Brexit Democratic Unionist Party resigned in protest over the Northern Ireland Protocol.

LONDON — The new Brexit deal signed by the U.K. and the European Union on Monday was heralded as a "turning point" for Northern Ireland, but must still pass muster in Belfast.

U.K. Prime Minister Rishi Sunak received a boost on Monday, as opposition leader Keir Starmer confirmed that his Labour Party would vote in favor of the deal — known as the Windsor Framework — which sets out to address a plethora of problems caused by the Northern Ireland Protocol.

This all but secures its passage through the British parliament in Westminster, irrespective of an anticipated opposition from hardline Brexiteers within Sunak's own Conservative Party.

The sticking point could come from across the Irish Sea in Stormont, near Belfast, where the devolved Northern Ireland Assembly has been suspended for a year after the pro-Brexit Democratic Unionist Party (DUP) resigned in protest over the Northern Ireland Protocol.

The DUP argued that the checks on goods travelling between Great Britain — England, Scotland and Wales — had effectively placed a border in the Irish Sea, ostracizing Belfast from the rest of the U.K. and depriving it of autonomy because of its adherence to EU legal systems.

The issues arose from the fact that Northern Ireland, which is in the U.K. and therefore no longer in the EU, shares a land border with the Republic of Ireland, which remains in the European alliance.

The previous accord, negotiated and signed by former Prime Minister Boris Johnson, was also criticized for jeopardizing the Good Friday Agreement — a landmark peace deal that brought an end to three decades of sectarian violence in Northern Ireland.

In a statement Monday, DUP Leader Jeffrey Donaldson said that his party's opposition to the previous arrangement had been "vindicated" and thanked Sunak and his predecessors for their work at the negotiations table.

"In broad terms, it is clear that significant progress has been secured across a number of areas, whilst also recognising there remain key issues of concern," Donaldson said.

"There can be no disguising the fact that, in some sectors of our economy, EU law remains applicable in Northern Ireland."

He said that the DUP will need to study the detail of the Windsor Framework against the party's "seven tests" to determine whether it "respects and restores Northern Ireland's place within the United Kingdom."

The seven tests

The first of these "seven tests" requires a deal to honor the sixth article of the 200-year-old Act of the Union, which stipulates that all U.K. citizens should be "entitled to the same privileges and be on the same footing" — a principle the DUP said was clearly not the case in Johnson's deal.

The party also demanded that Northern Irish consumers and businesses should not be forced to purchase certain goods from the EU and not from Great Britain.

It also insisted on the abolition of a theoretical trade border mandating additional checks on goods travelling between Great Britain and Northern Ireland, in case they advance into the Republic of Ireland. All checks must be abolished to re-establish Northern Ireland's standing in the U.K. market, the DUP said.

The fourth condition was for Northern Ireland to have a say over rules allowing goods to move back and forth over the southern border to the Republic, and for no new regulatory barriers to arise between Northern Ireland and the rest of the U.K., unless agreed in Stormont.

The seventh demand was to preserve the "letter and spirit" of Northern Ireland's position in the U.K., as set out in the Good Friday Agreement. This requires majority consent among the people of Northern Ireland for any reduction to the country's status as an equal part of the U.K.

The Windsor Framework removes checks in goods moving from Great Britain into Northern Ireland that will remain in the U.K., and also enables the same tax and excise changes to apply in Northern Ireland as in the rest of the U.K.

It also introduces a new mechanism dubbed the "Stormont brake," which gives the Northern Ireland Assembly a say on whether changes to EU goods rules affecting Northern Ireland should apply. Pulling this brake would give the British government a veto over the adoption of a new EU rule.

"The Windsor Framework provides practical solutions to many of the obstacles posed to the free movement of goods under the Northern Ireland Protocol," said Gaurav Ganguly, senior director of economic research at Moody's Analytics.

"The deal signals an improvement in relations between the EU and the UK, but it may not be enough to satisfy Unionists in Stormont. Despite the deal, the risk of a political impasse in Northern Ireland remains."

Rumblings of discontent have already emerged within DUP ranks. DUP MP Ian Paisley Jr., son of the party's founder, told the BBC on Monday that his gut instinct was that the new deal "doesn't cut the mustard."

Should the DUP reject the deal, Sunak may have to return to the negotiating table, reigniting the uncertainty that has plagued the U.K. political and investment landscape for almost seven years.