Gavin Barwell, Theresa May’s chief of staff, said the prime minister had “perfectly well understood” the impact of trade checks in the Irish Sea – and the EU would not believe the claim now that it was “underestimated”.
“He and David Frost [the chief negotiator] are intelligent people,” Lord Barwell said, ahead of crisis talks with the EU.
“I find it inconceivable that they didn’t understand what they were signing up to. They would have been advised very clearly by the civil service about that.”
The attack came after Lord Frost backtracked on past claims that the Northern Ireland Protocol – which introduced a customs border on imports from Great Britain – had been “a great deal”.
Instead, he said: “We underestimated the effect of the protocol on goods movements to Northern Ireland, with some suppliers in Great Britain simply not sending their products because of the time-consuming paperwork required.”
The issue has come to a head with so-called “sausage wars” over a looming ban on the sale of chilled meats to Northern Ireland from 1 July – which the UK signed up to.
But Lord Barwell warned the government: “I don’t think the EU is ever going to think that is credible.
“They know the quality of the civil servants involved in that work, and they know that British ministers would have been advised in detail on the implications of what they were signing up to.
“So I don’t I don’t think anyone who’s involved in the process is going to find it credible that the government signed up to something and didn’t understand what the consequences of that were.”
The UK is facing the threat of a trade war with it biggest market unless it stops backsliding on implementing the Protocol, as the EU sees it.
Nathalie Loiseau, the former French European affairs minister and now an MEP, warned it has the power to impose tariffs and quotas on British exports, unless there is full compliance.
However, the UK appears to be considering unilaterally extending a six-month grace period on the chilled meats ban, which the environment secretary branded “bonkers”.
Lord Frost has been meeting Maroš Šefčovič, the EU commission vice-president, both about the Protocol controversy and to discuss the Christmas Brexit trade agreement.
Mr Šefčovič warned that the EU would act “swiftly, firmly and resolutely to ensure that the UK abides by its international law obligations”.
But Lord Frost hit back, saying: “Further threats of legal action and trade retaliation from the EU won’t make life any easier for the shopper in Strabane who can’t buy their favourite product.
“Nor will it benefit the small business in Ballymena struggling to source produce from their supplier in Birmingham.”
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