DUBLIN (Reuters) – Brexit negotiations can move to phase two next week only on the basis of the tentative Irish border agreement which collapsed on Monday, otherwise talks to break through the impasse will resume in the new year, Irish Prime Minister Leo Varadkar said on Wednesday.

The deal, which is required if talks are to move to the next phase, was agreed on Monday with Dublin’s blessing after negotiators offered a “regulatory alignment” on both sides of the border that Ireland has with the British province of Northern Ireland.

This meant that the border would be the only UK land frontier with the European Union after Brexit.

But a Northern Irish party, which props up British Prime Minister Theresa May’s minority government, vetoed the deal saying it could not allow any divergence in regulations between Northern Ireland and other parts of the UK.

“As far as we’re concerned and as far as the European Commission are concerned … we stand by the text that had been agreed on Monday,” Varadkar told parliament on Wednesday.

“It is the desire and ambition and wish of this government that we should move onto the phase two talks but if it isn’t possible to move to phase two next week because of the problems that have arisen, well then we can pick it up in the new year.”

Varadkar said he understood May had difficult political problems to manage, but it was up to Britain to come back to negotiators in Brussels and Dublin.

He said he would speak to May in the coming days.

EU leaders meet on Dec. 15 but Brussels says Britain must present its offer this week or it will be too late for a decision on whether sufficient progress has been made in phase one.

The leader of Northern Ireland’s Democratic Unionist Party Arlene Foster spoke to May on Wednesday and a spokesman for the party said “there was still work to be done” on any border deal.

However Dublin insists it will not accept any change to the substance of the agreement struck on Monday.

“This is an agreement that has gone through a huge amount of work over many, many, many months and it’s a documents overall that is very balanced and exceptionally carefully constructed,” Irish Finance Minister Paschal Donohoe told a news conference.

“This is text that must be held and the meaning of that text cannot change. This is a position that the Irish government now has made very clear over the last number of days.”