She added the section in the agreement that ensures an open border between Northern Ireland, which is part of the UK, and EU member Ireland, is "essential to protect peace and stability on the island".
Her comments followed a report in the Financial Times that the British government is planning domestic legislation that would effectively override the international treaty obligations enshrined in the Brexit withdrawal agreement, particularly over issues related to the Irish border.
Johnson has said Britain could walk away from the talks within weeks and insists that a no-deal exit would be a "good outcome for the UK". He said in a statement that any agreement must be sealed by an EU summit scheduled for October 15.
The EU has previously said negotiations must conclude before the end of October.
The developments prompted widespread selling of the British pound as traders priced in a growing likelihood that the trade talks could be heading for collapse. The pound was down 0.9 per cent against both the US dollar and the euro.
During this transition period, the UK remains within the EU's economic orbit, benefiting from frictionless and tariff-free trade. But British businesses are worried about the uncertainty of what happens to their trading ties with their biggest market beginning January 1.
The EU's chief negotiator in the talks, Michel Barnier, said today that he will be seeking clarification from David Frost, his counterpart in the UK, "to better understand the government's intentions".
"This protocol is a condition for preserving peace and for protecting the integrity of the single market," Barnier told French radio France Inter. "It's also a pre-condition for confidence between us because everything that has been signed in the past must be respected."
Britain's Environment Secretary, George Eustice, sought to downplay concerns that the Government is seeking to tear up its treaty obligations. He argued that the Internal Market Bill, due to be published on Thursday, aims to tie up some "loose ends" where there was a need for "legal certainty".
He insisted that Johnson's government remained committed to the principles of the deal, which will see customs checks on some goods moving from the rest of the UK to Northern Ireland.
"What we are talking about here is what type of administrative customs processes you might have for goods that might be at risk of entering the EU single market," he told BBC radio.
James Slack, the prime minister's spokesman, also insisted that the Government remains "fully committed" to the Brexit withdrawal agreement and that this week's legislation is intended to clear up "ambiguity" and avoid "unintended consequences" in the complex border agreement.
If the UK to walk away from the talks, the two sides would be heading towards a no-deal outcome that would see tariffs and other impediments to trade imposed at the start of the year.
Kallum Pickering, senior economist at Berenberg Bank, said in that situation the costs would fall "disproportionately" on the UK, potentially tipping it back into recession in early 2021, while temporarily slowing the EU recovery.
"For the UK and the EU to find compromises on the sticking points, both sides must work to build trust," he said. "The UK's actions are doing the opposite."
The trade discussions have made very little progress over the northern summer, with the two sides seemingly wide apart on several issues, notably on business regulations, the extent to which the UK can support certain industries and over the EU's fishing rights access to British waters.
The German government said a deal was still possible and that it's in the interest of both sides to secure one.
"For this, Britain in particular needs to move on the core issues of governance, what's summed up as 'level playing field,' and on the issue of fisheries," said German spokesman Steffen Seibert.
- AP
