The Northern Ireland Protocol was signed separately from the Brexit trade deal between the UK and the European Union in order to avoid a "hard" border on the island of Ireland.
Physical infrastructure on the border between the province and EU member state Ireland was a regular flashpoint during three decades of violence over British rule in Northern Ireland.
Removing customs posts and allowing the free movement of goods and people between the north and south was a key part of the 1998 Good Friday Agreement that brought peace.
But it has complicated the post-Brexit situation.
To keep the border open, Northern Ireland is effectively still in the European single market -- but checks are required on goods heading east-west from mainland Great Britain.
Imposing controls was designed to prevent unchecked goods heading into the single market and customs union from England, Scotland and Wales via Northern Ireland.
'Not sustainable'
London has indefinitely suspended implementing those checks, and says it is ready to scrap the pact unless Brussels agrees to changes.
The EU is insistent the UK must abide by the terms of the deal it signed up to, warning it risks a possible trade war with the bloc if it unilaterally suspends any of the protocol.
But Brussels has made concessions, including proposing fewer checks on goods heading east-west to Northern Ireland.
The UK and the EU are set to resume talks but appeared to be on a collision course; Prime Minister Boris Johnson on Tuesday told his Irish counterpart Micheal Martin that the protocol was "not sustainable in its current form."
European leaders have insisted the terms will not be renegotiated.
In a growing dispute, The Times reported that British Foreign Secretary Liz Truss could scrap large parts of the deal from British law as early as next week.
The protocol has a suspension clause that allows either side to take "appropriate safeguard measures" if there are "serious economic, societal or environmental difficulties."
Brussels has warned of "serious consequences" if London triggers the clause, raising the prospect of a trade war of tariffs and barriers between the two sides.
The United States, which helped broker the Good Friday Agreement, and Irish-American President Joe Biden are watching closely, warning the UK to stick to what it signed.
Washington has warned that London can forget about a trade deal if its actions in Northern Ireland threatens the still-fragile peace.
Politics and business
The protocol has created political tensions in Northern Ireland between pro-British unionists and republicans who want the province to become part of Ireland.
There has already been some violence from hardline unionists, prompting fears of a return to Northern Ireland's troubled past.
The province's unionist parties oppose the protocol, warning it threatens Northern Ireland's status within the UK.
They have vowed not to nominate ministers to the Northern Ireland Assembly in Belfast until the protocol is scrapped.
That has raised the prospect of political paralysis at the legislature after Sinn Fein nationalists became the biggest party for the first time in Thursday's elections.
Sinn Fein leader Michelle O'Neill, who is set to become Northern Ireland's first minister, has warned she "will not be held to ransom" by Brexit disputes.
Yet many businesses in Northern Ireland, where a majority voted to stay in the EU, welcome continued access to the single market -- which was the default before Brexit.
On the mainland, suppliers are disgruntled by increased paperwork that has caused delays and even shortages of some products in Northern Ireland.
Grace periods have been extended several times for some products, including chilled meats and medicines.
The EU says it wants to find practical solutions to the problem. London says a yawning gulf remains between the two sides' positions.
The UK government's public budget forecasting agency has estimated that Brexit could lead to a 4% hit on GDP in the long term.
