The draft Brexit deal between Britain and the European Union was made possible by a breakthrough over the Irish border, but it could still prove the issue that unravels the agreement.
Both sides agreed last year to keep open Britain's land border with EU member Ireland no matter what happens but were stuck for months on how this could be achieved.
The end goal is to agree to a wide-ranging trade deal that removes the need for frontier checks on goods crossing between British Northern Ireland and the Republic of Ireland.
London hopes to secure this during a transition period planned to run from Brexit day on March 29, 2019, to December 31, 2020, in which Britain's relationship with the EU remains mostly the same.
But it has agreed that after then, a "backstop" arrangement would come into effect that keeps the border open unless and until the trade deal is struck.
Many commentators believe the backstop could become the basis of future EU-UK ties, which is why it has been subject to such intense scrutiny.
Why is this an issue?
After Britain leaves the European Union, the Irish border will become an external EU frontier.
Britain wants to leave the bloc's customs union and the single market, meaning checks would be required on people and products crossing from one territory to the other.
But both London and Brussels have pledged to avoid any physical infrastructure, or "hard border".
Many fear it could upset the delicate peace process that ended decades of violence between Protestant supporters of British rule over the province, and Irish Catholics nationalists, who believe in a united Ireland.
Residents and businesses on both sides of the now largely invisible 300-mile (500-kilometre) frontier also emphasise the importance of maintaining the free flow of trade and passenger traffic.
Is there a security risk?
British and Irish army checkpoints along the border were removed after the 1998 Good Friday peace accords, which largely ended three decades of conflict in Northern Ireland in which around 3,500 people were killed.
During "The Troubles" the border was a flashpoint for attacks and a lucrative smuggling route that helped fund paramilitaries.
Police have warned that any new infrastructure along the border could become a target for paramilitary activity by dissident militants who have not signed up to the peace deal.
What did the two sides propose?
The EU's proposed backstop envisaged keeping Northern Ireland in elements of the bloc's single market and customs union, meaning it would have to accept EU rules on quality standards and apply EU tariffs.
British Prime Minister Theresa May and the Northern Irish party that supports her government, the Democratic Unionist Party (DUP), said this was unacceptable.
They argued it would effectively carve off Northern Ireland from the rest of the country and create a border in the Irish Sea.
The British mainland accounts for 58 per cent of Northern Ireland's sales outside the province, nearly four times the value of exports to Ireland in 2016.
May proposed instead a temporary customs arrangement between the EU and the whole of Britain, including Northern Ireland.
What have they agreed?
The text has not yet been published but reports suggest both London and Brussels have given ground.
The EU has reportedly accepted the UK-wide customs backstop, but the deal includes provisions for Northern Ireland to stay more closely aligned with the bloc.
This could mean that some checks will be needed on goods transiting between Northern Ireland and the rest of Britain.
In return, Britain has agreed to a "level playing field" with the EU on state aid, competition, environmental and workers' rights, to ensure its businesses do not undercut those from the rest of the bloc.
The deal also includes some kind of review mechanism, although it reportedly falls short of Britain's early demands for either a time-limit on the arrangement or a unilateral right to leave it.
What has been the reaction?
European states will be scrutinising the draft text carefully for any signs that the backstop undermines the bloc's cherished single market.
In London, the DUP has warned it will not accept a situation where Northern Ireland follows trade rules set by the EU, nor one that places new barriers to mainland Britain.
Eurosceptic members of May's Conservative party also fear they will be trapped in a customs union with the EU for years after Brexit, unable to make their own trade rules.
The end goal is to agree a wide-ranging trade deal that removes the need for frontier checks on goods crossing between British Northern Ireland and the Republic of Ireland.