The president of the European Commission has threatened to not do business with Britain if "trust" is broken by the UK reneging on the Brexit Withdrawal Agreement.
Ursula von der Leyen said respect for the divorce treaty that paved the way for the UK to leave the EU on January 31 this year was a "prerequisite" for any free trade agreement with Brussels.
Boris Johnson is planning to use the internal market bill, due to be published on Wednesday, to renege on provisions in the Withdrawal Agreement, according to reports on Sunday.
If the bill does undermine commitments made in the Brexit divorce treaty, it will make it extremely difficult politically for Michel Barnier to offer the UK any concessions in the trade negotiations, which restart on Tuesday. Ministers have denied that the Government plans to tear up the treaty (see video below).
Mrs von der Leyen tweeted: "I trust the British Government to implement the Withdrawal Agreement, an obligation under international law and prerequisite for any future partnership. [The] Protocol on Ireland/Northern Ireland is essential to protect peace and stability on the island and the integrity of the Single Market."
Mr Barnier told French radio on Monday morning that he would ask David Frost, the UK's chief negotiator, for an explanation when they meet in London on Monday evening ahead of the trade talks.
The EU's chief negotiator said honouring the Withdrawal Agreement was "a precondition for confidence between us because everything that has been signed in the past must be respected".
He accused Britain of trying to get "the best of both worlds" but warned that there would not be an agreement "to the detriment of the Single Market".
"If there is no free trade agreement, the UK will fall back on traditional and customary WTO rules. There will be customs tariffs for products that we export to the UK and we will impose tariffs on products we import from the UK," Mr Barnier said.
A European Commission spokesman said: "While we are determined to reach an agreement with the UK, the EU will be ready in the event of a no-deal scenario to trade with the UK on WTO terms as of January 1, 2021.
"The full implementation of the Withdrawal Agreement and in particular the protocol on Ireland and Northern Ireland are essential these are legal obligations under international law. This is a matter of trust. This is a prerequisite, a precondition for the negotiations on the future partnership, and I think that's clear."
"We might be losing the UK, but we won't lose our stiff upper lip #Brexit," Charles Michel, the president of the European Council, tweeted in an effort to show that the EU would not buckle to UK pressure.
Mr Barnier said the bloc would not sign any trade deal unless there was an agreement granting EU boats access to UK fishing waters after Brexit, adding: "Excluding European fishermen is not acceptable to us. There are people who do not wish us well, who want to destroy us. Mr Farage and his friends."
Mr Johnson is expected to say later on Monday: "The EU have been very clear about the timetable. I am too. There needs to be an agreement with our European friends by the time of the European Council on October 15 if it's going to be in force by the end of the year.
"If we can't agree by then, then I do not see that there will be a free trade agreement between us and we should both accept that and move on."
The EU has already set a deadline of the end of October for the trade deal to be finalised so there is enough time to ratify it before the end of the year, when the UK will leave the transition period and the Single Market and Customs Union.
Failure to agree a deal will mean both sides trading on WTO terms, which, unlike the proposed agreement, will mean tariffs and quotas on goods.
Germany, which holds the rotating presidency of the EU, called on Britain to compromise over fishing and state aid.
"Germany is convinced that it is still possible to reach a Brexit agreement with Britain, but there is not very much time left," a German government spokesman said. "The United Kingdom must make compromises in the Brexit talks."
An EU diplomat told The Telegraph that a no deal outcome would make the UK the only country in the world without any agreement with the bloc. The source said leaving deadlocked issues such as the UK's future state aid regime and fishing rights to the last moment risked an accidental no deal.
"Boris Johnson said he wanted to put a 'tiger in the tank' of the negotiations. It looks like the tiger in the tank has shrivelled into a salted slug," said an EU diplomat.
"There is still time to put a deal in the offing, but at this moment it certainly doesn't look like it should be ready in time. If the UK no longer sees the merit in a deal, that would be unfortunate. It would make Britain the only country in this part of the world that doesn't have a relationship with the EU in one form or another."
MEPs in Brussels attacked Mr Johnson. Philippe Lamberts, a member of the European Parliament's Brexit Steering Committee, said the UK would be "a rogue state" if it reneged on the Withdrawal Agreement.
Manfred Weber, the leader of the largest political group in the parliament, tweeted: "Prime Minister, there is no such thing as a good outcome in Brexit. Instead of taking Northern Ireland hostage again, it would be better that you keep your word and stand by the Withdrawal Agreement. Can we trust you to keep your word?"