London - How to keep the Irish border free-flowing has proved to be the toughest issue to resolve in negotiating Britain's exit from the European Union.
Goods and people freely cross the land border between British Northern Ireland and the Republic of Ireland, as both countries are members of the EU.
The withdrawal agreement negotiated last year between London and Brussels contains a "backstop" plan to maintain this situation whatever happens with Brexit.
However, British MPs have rejected it three times and new Prime Minister Boris Johnson warns the backstop must go or Britain will leave the EU on October 31 without any deal.
Key facts on the Irish border
- Why is this an issue? -
As members of the EU, Britain and Ireland are both part of the bloc's single market and customs union, and their 300-mile (500-kilometre) border is largely invisible.
Local businesses and individuals cross over numerous times a day to transport goods, to go to school or work.
After Brexit, the border will become part of the EU's external frontier, suggesting checks will need to be made on products coming in and out.
But free movement is considered vital to maintaining peace in Northern Ireland, which was for decades plagued by violence over British rule.
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During "The Troubles", in which around 3,500 people were killed, the border was a flashpoint for attacks and a lucrative smuggling route that helped fund paramilitaries.
British and Irish army checkpoints were removed after the 1998 Good Friday peace accords.
Police have warned that any new infrastructure along the border could become a target for dissident militants.
In December 2017, London and Brussels agreed to "the avoidance of a hard border, including any physical infrastructure or related checks and controls".
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- What was agreed? -
In November 2018, the EU and Britain concluded a withdrawal agreement covering various issues related to the Brexit separation, including the Irish border.
The agreement set out plans for a post-Brexit transition phase until December 2020, during which Britain's relationship with the EU would remain unchanged in practice while the two sides agreed new arrangements.
Britain wants to resolve the Irish border issue as part of a new trade deal with Brussels, but this could take years, well beyond the transition phase.
As a result, it agreed to a backstop, or insurance plan, which guarantees an open border unless and until the new arrangements are in place.
The backstop plan would keep Britain aligned with rules of the EU's customs union, while Northern Ireland would adhere to some rules of the single market, such as on agricultural production.
- What is the problem?
The EU made a considerable compromise with the backstop, which allows Britain to remain in its customs union indefinitely without being a member of the bloc.
But in London and Belfast, many view it as a trap, and the backstop is the main reason that the British parliament repeatedly rejected the withdrawal agreement.
London and Brussels have agreed to put all their efforts into finding alternatives, but neither can end the backstop unilaterally, nor is there a time limit.
Eurosceptics fear it will keep Britain close to the EU for years after Brexit.
Meanwhile in Belfast, Northern Ireland's Democratic Unionist Party (DUP) fears the backstop creates a separate economic status from the rest of the United Kingdom.
The DUP has huge influence in London, as the party that props up the Conservative government.
- What now?
Johnson is pressing for the backstop to be removed from the withdrawal agreement, and the issue of the Irish border resolved after Britain leaves the EU.
He and fellow Brexit supporters in London say any checks can be carried out through "alternative arrangements" away from the border, including using new technology.
Brussels has committed to looking into alternatives that would mean the backstop never comes into force, but says there is no current solution and so it must stay.
Without agreement, Johnson says Britain will leave the EU regardless on October 31, risking huge disruption.
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By Alice Ritchie