Michel Barnier has warned that many of the new regulatory frictions hampering cross-Channel trade will be impossible to smooth over, as the inevitable consequences of Brexit begin to manifest themselves for businesses across Europe.

The EU’s Brexit negotiator said that some things have “changed for good” as a result of the UK’s policy choices, noting that “there are mechanical, obvious, inevitable, consequences when you leave the single market and that’s what the British wished to do”.

In an interview with a group of reporters, he underlined that Britain risks losing its tariff-free and quota-free trading rights with the EU if it diverges too far below European regulatory standards, singling out a recent controversy over a UK government decision to authorise a previously banned pesticide.

Two weeks after the end of Britain’s Brexit transition period, industries are still coming to terms with changes to their trading conditions, amid concerns in Whitehall that port disruption threatens supermarket supply shortages, and with British sectors from seafood exporters to professional musicians struggling to navigate the new barriers to accessing the EU.

Mr Barnier said that some upheaval since January — such as businesses curtailing cross-Channel trade as they navigate new paperwork — reflected adaptation issues that were leading to “glitches, problems, breakdowns” that should be cleared up over the coming weeks and months.

But he was also clear that both sides need to learn to live with the structural changes resulting from Brexit, ranging from checks on imports of farm produce to a ban on travellers bringing meat sandwiches with them when they cross the external border.

“This agreement will not be renegotiated, it now needs to be implemented,” he said.

Mr Barnier, who is due to step down from his position as chief negotiator in the next few weeks, specifically laid responsibility for any difficulties facing British musicians squarely at the door of the UK government.

Musicians including Laura Marling, Biffy Clyro and Dua Lipa have backed a petition calling on the British government to negotiate “a free cultural work permit” with the EU, because of concerns about post-Brexit limits on visa-free travel and requirements to comply with different EU member states’ labour market rules.

But Mr Barnier said the EU had proposed special travel rights for musicians as well as journalists and artists during the future relationship negotiations, only for the UK not to take up the offer.

“I very much regretted the fact that when it comes to mobility between the two sides that the British didn’t display any greater ambition. We had a number of initial proposals on this,” he said. “Of course, you have to be two to reach an agreement,” he said.

A proposal for the trade deal, published by the European Commission in March, offered enhanced visa-waiver rights for several categories of people including “sportspersons or artists performing an activity on an ad hoc basis” and journalists.

In comments supplied to music website NME earlier on Wednesday, UK culture minister Oliver Dowden said that Britain had sought “a mutually beneficial agreement that would have allowed performers to continue working and perform across the continent without the need for work permits” but “the EU turned it down, repeatedly”.

Mr Barnier also warned that Brussels would be “vigilant on all fronts” in policing how Britain implements the deal, which includes the right for either side to introduce tariffs if its companies are being placed at an unfair disadvantage.

Specifically noting “the debate over pesticides” in the UK, Mr Barnier said diverging EU and UK rules were a natural consequence of Brexit, but that “one ought to be careful . . . otherwise there will be consequences in terms of going on exporting without tariff without quota to our market”. He also warned that goods destined for the EU market had to comply with the bloc’s standards.

He noted the UK had reciprocal rights under the deal to pursue the EU.

Although it is not necessarily seen in Brussels as a sign of a wider deregulatory shift, the UK last week granted temporary authorisation for the use of a pesticide suspected of being harmful to bees because of concerns about a disease attacking sugar beets.

“Pesticides concern public health, the health of farmers, farm workers and consumers,” Mr Barnier said. “Depending on where you set the threshold in that area it can also have an impact on competition and competitiveness.”