The EU called at Brexit talks this week for Northern Ireland to effectively stay in a customs union with the bloc to prevent a hard border with Ireland, officials and sources said Friday.
The border between British-ruled Northern Ireland and Ireland is one of three key divorce issues that the European Union insists must be resolved before talks with Britain can move on to discussing a post-Brexit trade deal.
An internal EU document backed Dublin's position that Northern Ireland should obey the rules of the EU's single market and customs union to avoid the return of checkpoints on the border with the Republic of Ireland, sources told AFP.
Britain has insisted it will leave the single market and customs union in 2019, creating a puzzle about how to keep the open border that has helped underpin Northern Ireland's Good Friday peace agreement.
The document, quoted by Ireland RTE television, said it "seems essential for the UK to commit to ensuring no emergence of regulatory divergence from those rules of the internal market and the Customs Union."
The EU's single market ensures the free movement of goods, services, capital and people across borders within the 28-nation bloc, along with applying the same regulation to all businesses.
The customs union means that any goods entering the bloc from outside will not face any further border tariffs when they cross internal borders.
The demand is a further snag for this week's Brexit talks, which are also seeking to resolve the contentious issue of the bill the EU says Britain must pay to leave, and to secure the rights of the three million EU citizens living in Britain.
Ireland's government said it would not comment on internal documents but backed the EU's position on Northern Ireland staying in the customs union, using markedly similar language.
"The optimal solution would be for the UK to remain in the Customs Union and Single Market. But ultimately that is a decision for the UK," an Irish government spokesman said.
"Notwithstanding this, it is essential for the UK, at the very least, to commit to ways that ensure that a hard border is avoided and protect meaningful North-South cooperation and the all-island economy.
"This must include avoiding the risks presented by any regulatory divergence from the rules of the EU Internal Market and Customs Union."
The return of checkpoints on the 499-kilometre (310-mile) border between Northern Ireland and Ireland would be disruptive for the 30,000 people who cross it for work and cause extra paperwork for businesses.
There are also fears it could revive sectarian tensions that have been largely dormant since the 1998 Good Friday deal put an end to three decades of conflict, and boost smuggling, once a lucrative income for militia groups.