London - The main sticking point in Brexit negotiations is the future of the Irish border once Britain leaves the EU, with Brussels and London seemingly at loggerheads over the issue.
As EU leaders met in Salzburg, Austria on September 19 and 20 to discuss the status of the negotiations, here is an outline of the issue and what the two sides are saying:
Why is this an issue?
As things stand, when Britain leaves the European Union, the border between the British province of Northern Ireland and the Republic of Ireland will become an external EU border.
If Britain leaves the bloc's customs union and single market, as it says it wants to, border checks would be required.
But both London and Brussels have pledged to avoid any physical infrastructure, a so-called "hard border".
Map and key statistics on the border between the Irish Republic and Northern Ireland, part of Britain which is leaving the EU
Residents and businesses on both sides of the now largely invisible 300-mile (500-kilometre) border have also emphasised the importance of maintaining the free flow of trade and passenger traffic.
Around 31 percent of Northern Ireland's goods exports -- worth £2.4 billion (2.7 billion euros, $3.16 billion) -- went to the Republic of Ireland in 2016, the last year for which official data is available.
It is also estimated that 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 and Britain agreed in December last year to have a fallback plan or "backstop" that avoids a hard border if they cannot find any alternative solution to the issue.
Flow chart of suggested options for resolving the problem of the Irish border after Brexit
Brussels proposes that Northern Ireland retain elements of the EU's single market and customs union, including accepting EU rules on animal welfare and quality standards, and applying EU tariffs.
What does the British government want?
Prime Minister Theresa May says the EU plan would erect a "border in the Irish Sea" and undermine the unity of the United Kingdom.
Northern Ireland's biggest external market is Great Britain, accounting for 58 percent of sales outside the province.
These sales are worth nearly four times the value of exports to the Republic of Ireland in 2016, according to official data.
May proposes a broader plan for Britain as a whole to follow a "common rulebook" with Brussels to allow for frictionless trade in goods that would keep the Irish border open.
Is Brussels softening its position?
EU negotiator Michel Barnier has said Brussels wants to "improve" its proposal on Northern Ireland in a bid to break the deadlock.
"We can clarify that most checks can take place away from the border, at the company premises or in the market," he said.
"We need to de-dramatise the checks that are needed".
But the checks in ports and airports would remain, which Britain opposes.