The EU has warned Boris Johnson of “sanctions” if he fails to implement controversial goods checks in the Irish Sea after Brexit, opening up a fresh potential clash with the UK.

Brussels will “not tolerate any backsliding or half measures”, the senior adviser to chief negotiator Michel Barnier said – after the prime minister repeatedly claimed checks would not be necessary.

Stefaan De Rynck insisted the inspections were a joint legal agreement, as the price for Britain – but not Northern Ireland – breaking free of the single market and customs union.

“2020 will not just be about the future relationship, but also about implementing the withdrawal agreement – notably the Northern Irish protocol,” he told an event in London.

“We will have to be extremely disciplined to get it up and running in 11 months, to have the UK authorities apply the checks the UK has agreed to apply.

“That creates quite a lot of preparatory measures and, certainly on our side, we will not tolerate any backsliding or half measures.”

Questioned, by The Independent, Mr De Rynck declined to say what punishment the UK would face, but referred to “sanctions”.

However, it is understood that the EU will take the UK to the European Court of Justice (ECJ) if it fails to implement the checks, with the threat of heavy fines being imposed.

Crucially – and to the likely fury of Tory MPs – the ECJ will retain the power to fine the UK even after the transition period ends, at the end of 2020, when Brussels rule-making is meant to be over.

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Speaking at the 'Brexit - What Now?' event, at University College London, Mr De Rynck also:

* Warned the UK would also face action if it failed to secure the rights of EU citizens, asking, of the settled status scheme: “What happens to them if they don’t meet the deadline?”

* Argued 11 months was a very short time to avoid a no-deal Brexit in December, saying the EU had viewed the original 21-month transition as “rather short”.

* Insisted the EU would oppose “sector-by sector” deals, seen as the best way to avoid a December crash-out – because an “integrated pasta” was far preferable to “salami”.

* Said the UK had agreed to first strike a deal on allowing EU fishing in UK waters – and that “access to markets” could only come with “access to waters”.

Meanwhile, Dominic Grieve, the former Conservative MP and attorney general, warned of the danger of Mr Johnson embracing “populism” as “diversionary entertainment” away from future Brexit crisis.

He attacked Dominic Cummings, his chef aide as a “populist Maoist demagogue who believes in politics by vilification”.