The chairman of M&S has written to Brexit minister Lord Frost to warn that EU plans to end its stand-off with the UK over Northern Ireland threaten to add to, not ease, red tape.

Archie Norman said the Brussels offer “could result in worsening friction and cost and a high level of ambiguity and scope for dispute”, according to the letter first reported by the Financial Times.

Britain is seeking to rewrite the deal which left Northern Ireland within the EU single market for goods even after Brexit.

That protocol was designed to prevent the return of a trade border with the Republic – but has also effectively imposed a border between Britain and Northern Ireland in the Irish Sea.

The UK says the way the protocol is being applied by the EU is unsustainable and is threatening to trigger Article 16 – which would suspend parts of the Brexit treaty.

Britain and the EU have been locked in talks to try to prevent this scenario, seen as a last resort.

But M&S’s chairman has now expressed his concerns about a package of measures offered by Brussels offering to ease checks on goods coming from Britain into Northern Ireland.

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The proposals include labelling to ensure British products do not slip through the border into the Republic of Ireland, in the EU.

In his letter, Mr Norman said that labelling requirement would add £9m in extra costs annually for the 90m products the retailer ships to Northern Ireland from mainland Britain.

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‘Be flexible, find compromises’ for NI protocol

It also warned, according to the details reported by the FT, that under the proposals, a vet would still have to certify 95% of the food it sends across the Irish Sea, adding “a minimum of four hours per day” to transit times.

Altogether, and when including other checks, it would mean fresh goods taking 45 hours longer to reach stores than before Brexit, according to Mr Norman – and could result in M&S having to stop sending some product lines to Northern Ireland.

He said: “Detailed examination suggests to us that the proposals could end up being more costly to implement than full EU customs controls.”

Mr Norman called instead for a “risk-based regime” with limited checks on goods that would make use of digital technology.

It is not the first time that the M&S chairman, a former Tory MP, has used a letter to the government to weigh in on post-Brexit trade issues that are affecting the business.

In July, he warned of a political “running sore of symbolic proportions” if UK chilled products are banned in Northern Ireland and said consumers there were facing higher process and empty shelves as a result of the “pointless and byzantine” way rules were already being enforced.

Two months later, M&S announced that it was closing 11 stores in France blaming “lengthy and complex export processes” affecting fresh and chilled products following Brexit.

Half-year results published last week showed the group faced £13m in costs due to “ongoing EU border issues” largely related to its Republic of Ireland business.