Theresa May will warn Brussels she will not agree to “anything that threatens the constitutional integrity of the UK”, rebutting the suggestion Northern Ireland must continue to abide by EU regulations after Brexit.
The European Commission will today publish a draft withdrawal agreement that is expected to say Northern Ireland may need to be considered part of EU customs territory when Britain leaves the bloc.
The arrangement, which would be the so-called “default” option, would create a single regulatory space on the island of Ireland with no internal barriers.
Ms May has been told failure to agree on the legal text will stall talks on the future of the UK-EU relationship.
A Downing Street source said: “We are fully committed to implementing the December agreement, but the EU should be absolutely clear that the Prime Minister is not going to sign up to anything that threatens the constitutional integrity of the UK or its common market.
“Nor are we going to accept the ECJ as the final arbiter of the withdrawal agreement. There are many issues on which we are in agreement with the EU but, when it comes to these matters, you can expect the Government’s response to be robust.”
They added: “This is a draft negotiating position by the EU and not a final, binding text.”
Sources said the Prime Minister made clear in December that while there would be no hard border between Northern Ireland and the Republic, there would also be no divide in the Irish Sea.
The Foreign Secretary instead urged the Prime Minister to agree it should be the Government’s objective to prevent the crossing becoming “significantly harder”.
Supporters of a hard Brexit such as Mr Johnson fear keeping Northern Ireland in EU customs territory after Brexit would prevent Britain from breaking free from European economic structures and stop it from pursuing free trade deals elsewhere.
In the memo seen by Sky News, Mr Johnson wrote: “It would be wrong to see the Government’s task as maintaining no border.”
He said the task should be to “stop the border becoming significantly harder”, adding that – even if that happened – “95 per cent plus of goods” could be expected to pass through without checks.
Ian Murray, a Labour MP and supporter of the pro-EU Open Britain campaign, accused Mr Johnson of being ready to “undermine the Good Friday Agreement”.
“It can be little wonder that negotiations over Ireland are now severely strained, because Theresa May and her ministers have consistently failed to be honest about the implications of the hard Brexit they seek”, he said.
Following the leak, Downing Street dismissed the prospect of returning to a hard border, saying it had made clear it was not an option “on numerous occasions”.
Join our new commenting forum
Join thought-provoking conversations, follow other Independent readers and see their replies