Michel Barnier announced 80-85% of the treaty has been agreed and promised Brussels would respect the “constitutional integrity” of the UK.
But he vowed checks on animals and animal food products passing between Northern Ireland and Great Britain would have to increase tenfold.
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Currently 10% of these movements are screened, but Mr Barnier said bar codes on all lorries and ferries travelling between the two would have to be scanned.
“I understand those checks are politically sensitive but Brexit was the UK’s choice – not the EU’s,” he said.
Mr Barnier claimed the UK could eliminate those checks by choosing to remain in a customs union, but Theresa May has ruled that out.
Looking ahead to the next major staging post in talks, Mr Barnier said a deal was “within reach by Wednesday 17 October”.
Despite the warm words, he added that after Brexit “it won’t be business as usual”, and that even if a deal was reached businesses would have to change and adapt.
“Brexit has no added value – it is a lose-lose game,” he said.
On the back of Mr Barnier’s comments, the pound rallied by half a cent against the dollar to $1.32.
It also climbed to nearly €1.15, its highest level for almost four months.
Boris Johnson, the former foreign secretary and arch critic of Theresa May’s Chequers proposal on the future relationship, said the backstop “makes the UK a permanent EU colony”.
“This is far from take back control,” he wrote on Twitter.
“In the referendum, both sides said Leave meant leaving the customs union and single market.
“Yet this backstop inevitably means Chequers, staying in both, no say in either, and no right to escape.
“Number 10 must chuck Chequers now.”
Sky News understands senior ministers are being summoned for a meeting on Thursday to update them on the state of negotiations.
A similar meeting was held on Tuesday and a Number 10 source said the forum was not a decision-making body.
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It comes after Sky News revealed the Conservatives’ confidence and supply partner was prepared to vote against this month’s budget if its red lines for an EU deal are ignored next week.
DUP leader Arlene Foster said on the second day of a visit to Brussels that “any form of border in the Irish Sea will impede access for Northern Ireland to new UK trade deals” – removing “one of the key benefits of leaving the EU”.