Brexit negotiators are meeting now to try to resolve “big issues” ahead of a crunch summit of EU leaders this week.
The talks between the UK’s Dominic Raab and the EU’s Michel Barnier are focusing on Northern Ireland and future economic relations.
It comes as domestic political pressure on Theresa May increases.
Scottish Secretary David Mundell and Scots Tory leader Ruth Davidson say they will not accept Northern Ireland being treated differently.
In a letter to the prime minister, they said the integrity of the UK “remains the single most important issue for us” and cannot be undermined by any withdrawal agreement with the EU.
They indicate they will not tolerate a situation in which Northern Ireland remained in the customs union and single market, while the rest of the UK was outside it, or the introduction of customs and regulatory checks on goods moving between Northern Ireland and Britain that did not exist elsewhere within the UK.
A source close to Ms Davidson said the issue was a “red line” for her, while a source close to Mr Mundell told the BBC: “If you find yourself not agreeing with government policy” resigning would be the “logical outcome”. Read more