Negotiations over Britain’s future relationship with the EU appear on course for an acrimonious start after Michel Barnier poured scorn on Boris Johnson’s spokesman and suggested the new Northern Ireland secretary did not understand the withdrawal agreement.

Barnier said he expected the talks, starting on Monday, to be “very difficult” but pronounced Brussels as “ready” following the official sign-off by EU ministers of their instructions for their chief negotiator.

In an impassioned press conference during which he repeatedly banged on his lectern for emphasis, Barnier said the UK’s implementation of the agreement for avoiding a hard border on the island of Ireland was a prerequisite for any trade deal.

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Brandon Lewis, the Northern Ireland secretary, had claimed over the weekend there would be no border of any sort in the Irish Sea and promised “unfettered” trade after the UK leaves the single market and customs union at the end of the year.

“I think there are reasons for us to remain vigilant because the British minister in charge of Northern Ireland has come up with some very surprising statements,” Barnier said, noting the need for infrastructure to be erected for the necessary checks on goods.

“I would like him to take some time to read through the withdrawal agreement in some detail and he will see that commitments have been entered into by both parties to resolve what I call the squaring of the circle.”

The two sides had agreed in the withdrawal agreement – an international treaty – to enforce the the EU’s customs code on trade between Northern Ireland and Great Britain in order to avoid the need for a hard border with the Republic of Ireland.

Barnier also raised with reporters comments made on Tuesday by a Downing Street spokesman in which it was claimed that the goal of the UK in the negotiations was to secure British independence from the EU. Johnson’s chief negotiator, David Frost, has also in recent days spoken of the importance of the UK as a “sovereign equal” being untied from EU standards under a free trade deal.

Barnier said the EU was not seeking to retain any hold over the UK but that the prime minister had already committed to maintaining EU standards “less than six months ago”.

“Mr Johnson’s spokesman said the main objective of the UK in these negotiations is to ensure that we obtain the economic and political independence of the United Kingdom on 1 January this year, but, no, that is not true,” Barnier said. “The economic and political independence of the UK doesn’t need to be negotiated on its been done, it’s been achieved, that’s what Brexit has achieved. It was the will of the UK and they’ve left.

“Nobody’s going to discuss the sovereignty, the independence or the autonomy of the UK that’s not the purpose of these negotiations. I was very surprised to read this, supposedly something that was said by a spokesman from Downing Street.”

Barnier said the task was to put into legal text the commitments made by both sides in a 26-page political declaration on the future relationship, including the “robust” provisions to ensure Britain did not undercut the EU rulebook on workers’ and environmental standards and subsidies.

With reference to the prime minister’s refusal to extend the transition period beyond 2020, Barnier added: “If we want to succeed in the very short period of time that Mr Johnson has chosen we need to make sure we don’t start backtracking.”

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Under the EU’s vision of the future relationship, both sides will commit to not reduce their current standards. Brussels would retain the right to apply tariffs on the UK if there were “disruptions of the equal condition of competition” as EU law develops and the UK fails to follow that “reference point”. On state aid rules, the UK would need to “dynamically align” with EU law.

The UK will publish its vision of the future deal on Thursday but the government has said it will reject any deal that involves alignment on policy or Britain remaining under the jurisdiction of the European court of justice. The UK is insisting that a Canada-style trade deal, with looser provisions on ensuring a competitive level playing field, is the most appropriate option.

Barnier said the levels of trade with the UK were 10 times bigger than Canada. “At the same time Canada is some 5,000km away. It is clear that the rules cannot be the same.”

Earlier in the day, the Guardian had revealed that the EU is also demanding the UK maintains a ban on chlorinated chicken as the price for a trade agreement with Brussels, in a move that protects European meat exports and creates an obstacle to a deal with Donald Trump.

On the recommendation of France, a clause has been inserted into the EU’s negotiating mandate to insist that both sides maintain “health and product sanitary quality in the food and agriculture sector”, according to a copy leaked to the Guardian.

At the weekend, George Eustice, the UK environment secretary, refused to guarantee that the government would not allow the importation of chlorine-washed chicken as part of a trade deal with the US.

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The EU’s position was backed by the National Farmers’ Union on Tuesday whose president, Minette Batters, said it would be “morally bankrupt” and “insane” of Johnson to drop the UK’s high farming and food standards in order to strike a US deal.

She raised concerns that Johnson had dismissed misgivings over US animal welfare and food standards as “hysteria” and “mumbo jumbo” in a keynote speech in Greenwich three weeks ago.

“To sign up to a trade deal which results in opening our ports, shelves and fridges to food which would be illegal to produce here would not only be morally bankrupt. It would be the work of the insane.”