DUBLIN/BELFAST (Reuters) - The Irish government on Monday said an agreement that would leave Northern Ireland within the European Union’s customs union after Britain leaves the EU was “legally firm,” but Northern Ireland’s largest party said the issue remained open for debate.

The future of Ireland’s border with Northern Ireland - which will be the EU’s only major land border with Britain after Brexit - was widely seen as the biggest obstacle to an agreement on a 21-month transition deal to avoid a “cliff edge” Brexit.

But Britain and the EU on Monday announced a deal on a transition agreement without directly addressing disagreements about the border.

The Irish government said a deal agreed in December and fleshed out by Brussels three weeks ago still stood, despite British Prime Minister Theresa May’s rejection of it at the time as threatening the constitutional integrity of the United Kingdom.

Under the EU proposal, if all other attempts to avoid a hard border fail, Northern Ireland would form “common regulatory area” with Ireland and 26 other EU states, in effect keeping the British province in a customs union with the EU.

“The backstop is as legally firm as the government said it would be in December,” a spokesman for Prime Minister Leo Varadkar said. “The backstop applies unless and until something better is agreed.”

Northern Ireland’s largest Irish nationalist party shared the Irish government’s interpretation.

“Despite denials from Theresa May, there is now confirmation that the British government is accepting the agreements made, including the backstop option which would see the north remaining in the customs union and significant elements of the single market,” Sinn Fein Member of European Parliament Martina Anderson said in a statement.

But the Democratic Unionist Party, whose support May depends on to pass legislation in parliament, said it was not concerned as no deal had been done.

“There is nothing from today’s announcement to concern us. The border issue has not been resolved at this stage and we didn’t expect it to be,” a party source told Reuters.

Like the British government, the DUP believes a close post-Brexit EU-British free trade deal will allow for an open border, rendering the backstop unnecessary.

In a statement the party said the EU’s backstop scenario remained “totally unacceptable.”

“Work continues on the issues around the border in the coming months,” the statement said.