London: The 499 kilometre border between the Republic of Ireland and Northern Ireland is of little economic significance but looms as a growing obstacle to Britain's negotiations to exit from the European Union.
What sort of boundary emerges from Brexit is fraught with political, social and historical overtones and is complicated by the weakness of the governments trying to placate their own divided constituencies.
Ireland insists on a written guarantee there will be no "hard" border with Northern Ireland, something it says can be achieved only if Britain remains within the European single market or customs union. A hard border would involve physical check points and potential searches of cross border traffic, reminiscent of the bloody days of the "troubles" when republicans and loyalists waged a low-level war for 30 years from the late 1960s.
Liam Fox, Britain's international trade secretary, says a final position on Ireland cannot be reached until the "end state" of the UK-EU relationship after Brexit is defined.
Dublin has upped the ante and called on British Prime Minister Theresa May to accept a solution in which either the whole of the UK or just Northern Ireland remain in the single market and customs union.
Arlene Foster, Northern Ireland's conservative Democratic Unionist Party leader backs Britain's stance: "This is the maximum point of leverage for the Republic of Ireland. It's a stand-off. You cannot talk about the detail of the border until you get into the trading relationship. It's artificial to say you are going to sort out the border and then talk abut trade - the two are interlinked."
Although all are talking tough, none of the parties is acting from a position of strength.
May heads a minority government in part dependent upon the support of the Foster's DUP. May's party is divided over Brexit and how to negotiate the divorce from Europe.
Irish Prime Minister Leo Varadkar may face a snap election before Christmas over a crisis involving a police whistleblower that has brought his minority government to the brink.
Varadkar has two days to end the stand-off with the party propping up his government before it submits a motion of no confidence in his deputy prime minister.
The crisis erupted less than three weeks before a summit on Britain's plans to leave the EU, where Ireland will play a major role in deciding whether the negotiations can move onto the next phase.
Talks between Varadkar and Micheal Martin, the leader of the main opposition party, Fianna Fail, were continuing ahead of Martin bringing the motion of no confidence in Deputy Prime Minister Frances Fitzgerald before Parliament on Tuesday.
"The Taoiseach [prime minister] is doing everything he can to avoid an election, and hopes it will be possible to reach agreement with Micheal Martin," Varadkar's spokesman said in a statement on Sunday.
Ireland's concern with Britain's wish to leave the single market is that Northern Ireland would no longer be subject to EU rules, meaning all its exports would need to undergo EU inspection and tariffs, requiring a hard border. The only way to avoid this, they argue, is to keep Northern Ireland inside the EU single market and customs union, moving the hard border into Britain, at the Irish Sea.
Britain disagrees that a hard border is needed. The UK has suggested instead that large, cross-border businesses be subject to a "trusted traders" regime, which would allow inspections to take place on business premises, rather than at the border. For small traders, Britain's suggestion is even more radical: they should simply be exempt from the demands of inspections and tariffs.
The EU opposes such a notion.
Donald Tusk, the president of the European Council, has warned May she has just days to reach a deal if she wants to make a Brexit breakthrough at the European Council summit on December 14.
Britain's Fox says it will not meet the EU's 10-day deadline to resolve the Irish border issue, accusing Dublin of "blackmail" by trying to force Northern Ireland to stay outside the UK after Brexit.