A Government proposal to create a buffer zone at the border to break the Brexit deadlock has been rejected as "half-cooked" by the DUP.
The party called on London to "put down its foot" in negotiations with Brussels as the plan reportedly drawn up by Brexit Secretary David Davis was revealed yesterday.
Sinn Fein dismissed the alleged plan as "pie in the sky thinking completely divorced from the realities on the ground". The Alliance Party said it sounded like something which would be reported on April Fool's Day.
Under the radical blueprint, Northern Ireland would have a joint regime of UK and EU customs regulations, allowing it to trade freely with both.
A 10-mile wide "special economic zone" would be created along the 310-mile border, allowing local traders to operate under the Republic's rules, thus avoiding checks.
The proposal was based on the model in place in Liechtenstein, but Downing Street last night denied that it was a runner.
"The Prime Minister has been absolutely clear that we cannot and will not accept a customs border down the Irish Sea, and that we will preserve the constitutional integrity of the UK's common market," a spokesperson said.
DUP MP Sammy Wilson said the Government had not discussed the latest reported proposal with his party.
Describing it as "at best contradictory", he said it had arisen only because London had failed to "make it clear to the EU that regardless of (Michel) Barnier and EU negotiators' attempts to keep us in the customs union and the single market, we are leaving".
Mr Wilson continued: "Instead of moving from one set of half-cooked ideas to the other, it is now time for the Government to put down its foot and make it clear to EU negotiators that the Prime Minister stands by her commitment that no deal is better than a bad deal.
"And if they want to avoid the consequences then they need to stop dismissing the perfectly feasible ideas that were put forward in August of last year."
Sinn Fein's Brexit spokesperson David Cullinane TD said: "Reports that David Davis is proposing a border buffer zone for the North would be ridiculous if they were not so serious.
"The issue of the border on the island of Ireland needs real ideas and real solutions.
"Davis and the British government are instead intent on bringing forward ideas they know to be unworkable."
Mr Cullinane said that we appeared no closer to an actual framework for a post-Brexit Northern Ireland, with the UK leaving the EU in 10 months.
"This is all the more reason for real progress on the issue in time for the June EU summit," he said. "Time is being lost by this bluster on buffer zones.
"The British need to get real. The Irish government for its part needs to hold firm and ensure there is no hardening of the border on the island of Ireland."
Alliance deputy leader Stephen Farry said: "This is June 1, not April 1. The UK Government doesn't even seem to understand the problem that they are allegedly trying to solve.
"The challenge is to avoid a border. Simplistically shifting the problem doesn't resolve it, but actually makes it worse. Any physical barrier on the island will bring political, economic and security implications.
"Alliance does believe Northern Ireland requires special economic arrangements in the context of Brexit. But a special economic zone with double regulation does not fit the bill."
An Ulster Unionist spokesperson said: "These proposals are totally impractical and unworkable and indeed would place a question mark over the constitutional position of Northern Ireland."
TUV leader Jim Allister (left) said: "I've heard some daft proposals, but this one takes some beating. It would fly in the face of the decision that the UK as a whole is leaving the EU.
"We cannot tolerate a situation where Northern Ireland would be left half-in, half-out. If that was the government's proposal, it's time that the government was gone."
Labour MP Chris Leslie, a supporter of the Open Britain campaign against a hard Brexit, said: "If there was an award for coming up with unnecessarily complicated and convoluted solutions to self-inflicted problems, David Davis would win it every year.
"The solution to this dilemma is staring David Davis in the face: the UK as a whole must stay in the single market and customs union."
Liberal Democrat Brexit spokesman Tom Brake said: "More and more by the day, Tory plans are sounding like something out of Alice in Wonderland.
"The public must be given the final say on the deal, with the opportunity to exit from Brexit."
A spokesperson for Taoiseach Leo Varadkar said: "At this stage in the process, the UK must engage in a more detailed and realistic way on the draft text of the protocol on Ireland and Northern Ireland, including the backstop."