A report drawn up by the Republic of Ireland’s customs authority has ruled out an open customs border with the North – effectively pouring cold water on UK plans for the frontier.

The unpublished report, drawn up after the Brexit vote and obtained by Irish broadcaster RTE, spells out the huge logistical difficulties Britain leaving the EU will cause for trade on the island of Ireland.

The UK says a “unique” solution can keep the customs border, which is currently not policed, frictionless – despite the UK’s intention to leave the EU customs union, which would ordinarily see checks placed there.

But the report, by the Revenue Commissioners, says: “It is probably somewhat naive to believe that a new and entirely unique arrangement can be negotiated and applied to the EI/UK land frontier.

“Once negotiations are completed ... the UK will become a third country for customs purposes and the associated formalities will become unavoidable. While this will affect all member states, the effect will be more profound on Ireland as the only EU country to have a land border with the UK.”

It continues: “This extra layer of formalities for movements that are currently intra-union movements will not only place a considerable administrative burden on traders, it will also have a negative impact on trade flows and delay the release of goods.”

The question of how to maintain a frictionless NI border while the UK leaves the customs union has perplexed Brexit negotiators. The EU has insisted the issue must be settled before trade talks can begin, alongside other separation issues such as citizens’ rights and the divorce bill. The UK has proposed an infrastructureless border based on spot checks. Both the UK and EU believe there should be no “hardening” of the border with the Republic.

A European Parliament motion carried last week suggested that Northern Ireland could stay in the customs union and single market, with customs checks moved to ports in the Irish Sea.

A spokesperson for Ireland’s Department of Finance has said the report was never finalised and “predates the many developments and papers that have issued this year”.

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"Thinking has now moved on very significantly. In May of this year, the Revenue Chairman told a Joint Oireachtas Committee that Revenue is not considering any new customs posts,” the spokesperson said. "The Irish government has consistently said that the best outcome would be for the UK to remain in the customs union and the single market.