“Backstop means backstop,” the EU’s chief negotiator told a press conference in Brussels, as he insisted the UK needed to be pragmatic and accept that Northern Ireland would have to be treated differently to the rest of the country, with checks taking place on goods travelling across the Irish Sea.

The backstop plan is set to snap into effect should a free trade deal or technological solution not be in place by the end of the transition period to prevent the need for a hard border between Northern Ireland and the Republic.

The EU has suggested that Northern Ireland effectively stay in the customs union and adhere to large parts of the single market rules in such a scenario, a proposal flatly rejected by Downing Street as it would create a border within the UK.

Barnier said the EU was willing to consider the prime minister’s suggestion – made in a document published on Thursday – that the UK stay in the customs union after the transition period to avoid some checks, although he said it could not be a “time limited” arrangement. But he would not countenance the UK staying in parts of the single market, as that would be cherry-picking access.

The UK has yet to say what it would do with regard to avoiding the border checks necessary on goods travelling in and out of the single market.

Barnier said, however, that there could be no suggestion of the UK as a whole taking advantage of the offer made for Northern Ireland to stay in regulatory alignment and therefore avoid the need for checks on goods travelling to and from the EU.

He detected, he said, a sense of of nostalgia among some British negotiators for the privileges of EU membership, but the bloc had been clear that it was not willing to grant “à la carte” single market access.

“Sometimes I sense in those proposals some sort of nostalgia at the moment of leaving the European Union, since you want to stay everywhere without ever again respecting the regulatory framework,” Barnier said, saying UK ministers including Boris Johnson, the foreign secretary, had put forward “all sorts of proposals that surprise us, that appear paradoxical.”

“We respect Britain’s red lines. I’d wish the British would respect their own red lines too.”

He added: “Let me be clear: our backstop cannot be extended to the UK. Why? Because it has been designed for the specific situation of Northern Ireland ... Let’s go back to pragmatism.”

On Thursday, David Davis, the Brexit secretary, who had at one point appeared on the brink of resignation, convinced May to include an expected timeframe for the backstop of December 2021.

Barnier, however, was withering about such a concept. “The time-limited term does not work for us,” he said.

No 10 immediately hit back at Barnier’s rejection of a UK-wide backstop, saying May “has been clear that we will never accept a customs border between Northern Ireland and the rest of the United Kingdom”.

The spokesman added: “That position will not change. We are also committed to maintaining the integrity of our own internal market. The commission’s proposals did not achieve this, which is why we have put forward our own backstop solutions for customs.”

Downing Street suggested Barnier’s comments were still up for debate. “All parties must recall their commitment in the joint report to protect the Belfast agreement in all its parts. Michel Barnier has confirmed today that discussions will now continue on our proposal,” the spokesman said.

The Democratic Unionist party MP Nigel Dodds said: “Michel Barnier’s latest comments demonstrate he has no respect for the principle of consent or the constitutional integrity of the United Kingdom. This is nothing more than an outrageous attempt to revert to the annexation of Northern Ireland. We will not accept such a proposal.”

In response to Johnson, who had suggested in leaked comments that a “meltdown” in the talks was coming and that the UK could benefit from the assertive negotiating style of Donald Trump, Barnier said: “I always listen very attentively to what he says and it is always very stimulating. But I am not going to comment on that.”

Barnier said he would not be intimidated by UK politicians. “If we want to build a new relationship, we need a base of trust, we also need more realism about what is and what will be possible and what is not.”

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