Brexit deal is a 'gentleman's agreement' says EU
The agreement to move Brexit talks on to the next phase is not strictly legally binding but the two sides have "shaken hands" on it, the EU has said.
The European Commission said the UK had endorsed the "gentleman's agreement" between David Davis and Michel Barnier.
On Sunday the Brexit Secretary said guarantees on the Northern Ireland border were not legally binding unless the two sides reached a final deal.
But he told LBC Radio on Monday they would be honoured whatever happened.
The BBC's assistant political editor Norman Smith said the Brexit Secretary's clarification – in which he insisted one of the government's key aims was to ensure that the Northern Ireland peace process was not harmed – came among concerns in Dublin about the enforceability of Friday's phase one agreement.
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Mr Davis said he had been taken out of context when he appeared to tell the BBC's Andrew Marr that guarantees designed to avoid a hard border on the island of Ireland were a "statement of intent".
"What I actually said yesterday in terms was, we want to protect the peace process, want to protect Ireland from the impact of Brexit for them," he said.
"I said this was a statement of intent which was much more than just legally enforceable. Of course it's legally enforceable under the withdrawal agreement but even if that didn't happen for some reason, if something went wrong, we would still be seeking to provide a frictionless invisible border with Ireland."
A European Commission spokesman said the first-phase deal on the Northern Ireland border, the divorce bill and citizens' rights did not strictly have the force of law.
"But we see the joint report of Michel Barnier and David Davis as a deal between gentlemen and it is the clear understanding that it is fully backed and endorsed by the UK government."
She added: "President Juncker had a meeting with Prime Minister May last Friday morning to ascertain that this is precisely the case. They shook hands."
A clause inserted in Friday's agreement at the insistence of the Republic of Ireland government says the UK would have full "regulatory alignment" with the EU in some areas if it leaves the EU without a deal.
Theresa May will tell MPs later there is a new "sense of optimism" in the Brexit talks after her last-minute deal aimed at moving them to the next phase.
She will say she expects EU leaders to agree to start talks about future trade and security at a summit on Thursday and insist she did not cave in to Brussels over the so-called divorce bill and "alignment" with EU laws.
The UK is due to leave the EU in March 2019 but wants a two-year transition period after that date, with continued access to the single market to allow businesses to adjust.
That is expected to be the focus of the next round of talks in Brussels in the new year, before discussions can start on a free trade agreement.