DUBLIN (Reuters) - Ireland said on Wednesday the British government and its Northern Irish allies must offer alternative proposals or risk triggering a backstop agreement which would avoid customs checks at the Irish border after Britain leaves the European Union.

Ireland's Taoiseach (Prime Minister), Leo Varadkar, speaks outside Stormont House, in Belfast, Northern Ireland February 12, 2018. REUTERS/Clodagh Kilcoyne

Brexit negotiators agreed in December that Northern Ireland, which is set to become the United Kingdom’s only land frontier with the EU, will in a worst case scenario stay aligned with the rules of the bloc’s single market and customs union once Britain leaves.

The agreement was put into legal effect in a draft treaty on Wednesday. But it was criticized by politicians in Northern Ireland as constitutionally unacceptable and British Prime Minister Theresa May said no government could ever agree to it.

“It’s not okay for people, whether pro-Brexit politicians in Britain or parties in Northern Ireland, to just say ‘no’ now. It’s incumbent on them, if they can’t accept the backstop, well then they must detail how Option A or B would work,” Irish Prime Minister Leo Varadkar told Ireland’s Newstalk radio station.

” And actually write them down, they can’t be theoretical stuff about congestion charges and tolling in another country,” he said, referring to British Foreign Secretary Boris Johnson’s suggestion that technology similar to that used for traveling between two London boroughs could apply to the Irish border.

Britain’s government, grappling with one of the most complex aspects of its departure from the EU, has promised to preserve the integrity of its own internal market and Northern Ireland’s place within it.

It says it does not want a hard border between Ireland and Northern Ireland, or physical border infrastructure of any kind across the 500-km (300-mile) frontier. It reiterated on Wednesday that it aims to avoid checks and controls there via a future EU-British economic relationship.

If this is not possible, Britain says it will propose “specific solutions to address the unique circumstances”.

Irish Foreign Minister Simon Coveney expressed frustration that nobody knew what either of those options looked like. He said Dublin was not looking to pick a fight but was crying out for some certainty with Brexit just over a year away.

In Brussels, the EU’s chief Brexit negotiator Michel Barnier denied that the contingency plan would loosen Northern Ireland’s constitutional ties to the rest of the United Kingdom, although he conceded there might have to be extra checks on trade between the two.

The Democratic Unionist Party (DUP), on whom May relies for a slim parliamentary ay majority to see through her Brexit legislation, reacted furiously, saying it would be economically catastrophic for Northern Ireland not to have unfettered access to the British market.

Even the smaller, more moderate Ulster Unionist Party voiced strong opposition, describing the draft treaty as “nothing short of a move by the EU to annex the British province”.

“I think we’re now facing the biggest test in the resolve and character of this government in terms of how you deal with the EU,” Ian Paisley, one of 10 DUP lawmakers who back May, told her Northern Irish minister at a parliamentary committee.

“I’d ask the government to show some teeth now to the European Union that we will not be rolling over to their demands about annexing part of our union.”