Turning up the pressure on the EU, Prime Minister Boris Johnson is threatening to table new legislation and override requirements for new Northern Ireland customs arrangements, meant to prevent the return of checks at the Irish border from next year.
Mr Barnier was asked about on reports that Mr Johnson planned to use the internal market bill, due to be published on Wednesday, to renege on provisions in the Withdrawal Agreement.
31 but talks have so far made little headway on agreeing a new trade deal by the time a status-quo transition arrangement ends in December.
The report came after Johnson said the October 15 European Council meeting was the ultimate deadline for any deal to be in place by January 1 next year.
Boris Johnson is drawing up legislation that will override the Brexit withdrawal agreement on Northern Ireland, a move that threatens the collapse of crunch talks which the prime minister has said must be completed within five weeks.
Senior members of Northern Ireland's Sinn Fein and SDLP parties, the region's two largest Irish nationalist groups, also criticised the UK's reported plan.More news: Trump Hired Obama Look-Alike In Bizarre Video Stunt
Britain may be posturing or sabre-rattling on Brexit, Tánaiste Leo Varadkar said, adding "no domestic law can trump an worldwide treaty".
"As a Government we are preparing, at our borders and at our ports, to be ready for it". The source said leaving of deadlocked issues such as the UK';s future state aid regime and fishing rights to last risked an accidental no deal.
Johnson said the country would "prosper mightily" even if Britain had "a trading arrangement with the European Union like Australia's" - the United Kingdom government's preferred description of a no-deal Brexit.
Frost and European Union negotiator Michel Barnier are due to meet in London on Tuesday (local time) for the eighth round of negotiations since Britain left the now 27-nation bloc on January 31. "We will have full control over our laws, our rules, and our fishing waters", he said.
Johnson's hardball tactics will likely compound suspicions from British pro-EU "remainers" that his Conservative government envisaged a "no-deal" scenario all along.
"The EU must also realize that we are serious about leaving with an Australian-style trading relationship and reclaiming our independence as a sovereign nation if we can not find acceptable terms", he said.More news: Cong Leader Kavitha Reddy Apologises for Attacking Actress Samyuktha Hegde
But Mr Johnson, in an apparent bid to focus minds as another set of talks gets under way on Tuesday, will say that there is "still an agreement to be had", one that is based on deals Brussels has previously struck with "Canada and so many others".
"Instead of taking Northern Ireland hostage again, it would be better that you keep your word and stand by the Withdrawal Agreement".
He told French radio that honouring Withdrawal Agreement was "a pre-condition for confidence between us because everything that has been signed in the past must be respected".
"There is no sense in thinking about timelines that go beyond that point", he will say.
And Simon Coveney, Ireland's foreign affairs minister, tweeted ominously: "This would be a very unwise way to proceed".
While some national capitals favour a tougher negotiating stance than the one being pursued by Barnier, they appear content to leave the talks in his hands, fuelling fears that there will be no deal if back channels are not created to test new ideas in confidence.More news: United contact Sancho as Liverpool prepare to sell midfielder to Barca