Leo Varadkar, Ireland's Prime Minister, has warned Theresa May that the EU will never accept a Brexit deal that allows Britain to unilaterally walk away from a hard home in Ireland.
Mr Varadkar told the Prime Minister that he was willing to consider a "review mechanism" for the so-called Northern Ireland backstop – the last big sticking point in Brexit negotiations – Britain could not have the right to end it on its own.
Mrs May convenes her cabinet on the State of Brexit Talks, with Downing Street still hopeful that a deal can be struck at a special EU summit. British officials are now eyeing a date in late November but have downplayed the prospects of a breakthrough this week.
The main sticking point is the Northern Ireland backstop, the undertaking by both sides to avoid a hard border in Ireland and to maintain the progress of the peace process.
Mr Varadkar's comments, made in a phone call to Mrs. May, are as much as possible to inject new impetus into the talks.
Dominique Raab, her Brexit secretary, who wants a plan that would have allowed Britain to pull out of the backstop with only three months' notice.
Mr Raab claimed in a letter to Mrs May that he could not help but support it in Dublin and Brussels, but on Monday it was clear.
Downing Street was again asked Mrs May supported Mr Raab's call for a unilateral right to pull out of the backstop, but the prime minister's spokesman said that Britain was only seeking a "mechanism".
Rumors swirled in Westminster that Mr. Raab, a committed Brexiter, might be about to quit, but his allies said the minister was feeling "relatively positive about things" and what "not on the brink of resignation".
They confirmed that while Mr. Raab believed in Britain's right to "unilaterally revoke the backstop", his principal concern what a clear way out of any temporary customs arrangement in Ireland.
Pro-European Tories believe Mr Raab is just positioning himself as a champion of a tough line with Eurosceptic Tories ahead of a final compromise. "It's pretty desperate stuff," said one.
Mrs May has made a significant concession on the backstop from the EU.
But Conservative Eurosceptics, including Mr. Raab and about nine other ministers in the cabinet, want to assure that this "temporary" arrangement does not exist.
Downing Street said after Mrs May's call with Mr. Varadkar: "In order to ensure that the backstop, if ever needed, would be temporary, the prime minister said that there would be a mechanism through which the backstopped could be brought to an end . "
The EU has insisted on a Northern Ireland-specific "backstop to the backstop" – an ultimate insurance guarantee against a hard border if all other options failed. This, together with the proposed exit mechanism, is one of the two big remaining issues in dispute over the backstop.
British officials maintain that the two problems are linked and can be solved at the same time. They could say that progress could be made quickly.
Tuesday's cabinet meeting is likely to end, despite Number 10's efforts to play down expectations of an early breakthrough. But a separate political declaration covering the EU / UK future relationship.