The main sticking point in Brexit negotiations is how to keep the land border between Britain and Ireland open after Britain leaves the European Union.
London believes frontier checks can be avoided through a new trade deal with Brussels, but accepts the need for a fall-back plan to address the issue until that deal is agreed.
The terms of the so-called backstop are disputed, however, and both sides are racing to resolve their differences ahead of a key EU summit next week.
Why is this an issue?
After Brexit, the border between the British province of Northern Ireland and the Republic of Ireland will become an external EU frontier.
Britain says it wants to leave the bloc’s customs union and 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, a so-called “hard border”.
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.
At least 30,000 people cross the border every day for work, with many residents of border regions living on one side and working on the other.
Is there a security risk?
British and Irish army checkpoints along the border were removed in the wake of a peace deal in 1998 known as the Good Friday Agreement, which largely ended three decades of conflict in Northern Ireland in which around 3,500 people were killed.
The border during The Troubles 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 on to the peace deal.
What is the EU proposing?
The EU’s backstop proposal would see Northern Ireland stay in elements of the bloc’s single market and customs union, meaning it would accept EU rules on quality standards and apply EU tariffs.
British Prime Minister Theresa May says this is unacceptable, saying it would effectively carve off Northern Ireland from the rest of the country by creating a border in the Irish Sea.
Northern Ireland’s biggest external market is Great Britain, accounting for 58 percent of sales outside the province — nearly four times the value of exports to Ireland in 2016.
The issue is particularly sensitive as May’s government is propped up by Northern Ireland’s Democratic Unionist Party (DUP), who reject any attempt to change the province’s status.
What does Britain want?
May’s backstop proposal is to create a temporary customs arrangement between the EU and the whole of Britain, including Northern Ireland.
But she suggests it should only be in place until the end of December 2021 — something Brussels has rejected, saying any fall-back plan cannot be time-limited.
Room for compromise?
EU negotiator Michel Barnier suggested that “most checks can take place away from the border, at the company premises or in the market”.
But the checks in ports and airports would remain, which Britain opposes.
The British government meanwhile has indicated it might be open to regulatory checks between Great Britain and North Ireland, noting some already exist, for example on agricultural products.
London has yet to publish plans addressing regulatory issues, for example rules on the temperature of milk traded across the Irish border.
However, the DUP has warned it will not accept either a customs or regulatory border between Northern Ireland and the rest of Britain.
