Britain has said there should be no border posts between Ireland and the British province of Northern Ireland after Brexit, in a paper that attempts to resolve one of the most complex aspects of its exit from the European Union.
Some 30,000 people cross the 500km border every day without customs or immigration checks; negotiators must work out how to tighten controls without inflaming tensions in a region that suffered decades of bloody turmoil before a peace deal in 1998.
As part of a series of papers that British Prime Minister Theresa May hopes will push forward talks with the EU, the government on Tuesday outlined its vision for a "frictionless" frontier without "physical border infrastructure and border posts".
May also said Britain would consider stepping in to replace some EU funding for peace projects in Northern Ireland after it leaves the bloc in March 2019 and beyond, to prevent a resurgence of violence between pro-British Protestants and Catholic Irish nationalists.
"Both sides need to show flexibility and imagination," said a British government source, who declined to be named.
"We have some very clear principles. Top of our list is to agree, upfront, no physical border infrastructure, that would mean a return to the border posts of the past and is completely unacceptable to the UK."
Commenting on an advance briefing of the position paper, the Irish government said it was "timely and helpful" and that it hoped enough progress could be made to move talks forward.
"Protecting the peace process is crucial and it must not become a bargaining chip in the negotiations," it said.
Northern Ireland sold $3.5bn of goods into Ireland in 2015, according to official figures, and many businesses have complex supply chains that involve crossing the border multiple times during the production process.
Britain said it wanted to maintain a Common Travel Area, a pact that allows free movement between the UK and Ireland for British and Irish citizens, and introduce new 'trusted trader' arrangements to help larger companies. Smaller firms would be exempt from customs processes.
It rejected the idea of an effective customs border in the Irish Sea that separates England, Wales and Scotland from Ireland and Northern Ireland as "not constitutionally or economically viable".
The border is one of three priority issues that the EU is insisting must be dealt with during the opening rounds of talks before moving on to Britain's future relationship with the bloc.