Stephen Barclay, the Brexit secretary, said negotiations had to “move forward at pace”, with the crucial summit at which an agreement must be reached just two weeks away.
He ducked a question about whether an EU refusal to begin talks so quickly – after its overnight criticisms of the prime minister’s blueprint – would mean the plan “is not going to fly”.
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No 10 has already hinted that the prime minister may boycott the Brussels summit on 17 October unless intensive talks begin straight away – which would bring any hope of a deal crashing down.
The EU is determined not to be blamed for such a failure, prompting Jean-Claude Juncker, the EU Commission president, to point to “positive advances” in the UK’s new plan.
However, behind the scenes, his negotiator Michel Barnier, is believed to have warned that handing Stormont a veto on whether to align with EU or UK regulations would leave the EU “trapped” and imperil the single market.
Meanwhile, MEPs are expected to announce that they will reject the plan in its current form as early as today.
Asked when formal negotiations would begin, Mr Barclay said: “We need to move forward at pace intensively.”
Asked if that mean Brussels must agree to start work on the blueprint by the weekend, he told BBC Radio 4’s Today programme: “Yes.”
The “two borders” plan – a customs barrier in Ireland, with inevitable checks, and oversight of the regulatory border between Northern Ireland and Great Britain – crosses EU red lines in several ways.
Tony Lloyd, Labour’s shadow secretary of state for Northern Ireland, said he was certain neither Brussels nor Dublin would compromise on something “so fundamental”.
“This is simply not Good Friday Agreement compatible. It simply doesn’t do that,” Mr Lloyd said.
And Trevor Lockhart, group chief executive at Fane Valley Co-op, an agri-food business in Northern Ireland, said its interests “have now been sacrificed”.
“As they currently stand, the government proposals don’t represent a final destination for Northern Ireland business,” he warned.
But Mr Barclay insisted the UK commitment of no customs-checking infrastructure at the border meant it stood by “our commitment to the Belfast Good Friday Agreement”.
And he dismissed fears about smuggling, saying: “It’s about what is proportionate. Most smuggling operations aren’t actually addressed at a border.
“Most smuggling is actually addressed in terms of intelligence, in terms of information-sharing, in terms of close partnership between the various agencies.”
The prime minister is expected to personally make a statement to MPs about his plans today, after earlier hints that he would leave the task to his Brexit secretary.