BREXIT could lead to fresh food from Scotland rotting as it sits in long queues waiting to be exported, the chief executive of Sainsbury’s has warned.

Mike Coupe said there would be a “detrimental” effect for British farmers and shoppers, unless the UK’s Brexit negotiators are able to agree a customs arrangement that keeps the established food supply chains.

The supermarket said: “The UK sources roughly a third of its food from the European Union and food is by far and away the UK’s largest export.”

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“If you take our fresh produce supply chains, for example, we put things on a lorry in Spain and it will arrive in a distribution centre somewhere in England, and it won’t have gone through any border checks.

“Anything that encumbers that has two effects: it adds cost, and it also has a detrimental effect on freshness – if you’re shipping fresh produce from a long distance, even a few hours of delay can make a material impact.”

Scottish food and drink exports to EU countries were worth £2.3 billion last year, up £133 million.

Just last week, the British Retail Consortium warned of rising food prices and reduced choice on the shelves of Britain’s supermarkets after the UK leaves Europe in just 18 months’ time.

Coupe said he did not believe the government “fully recognised” the repercussions of supply chain disruption.

Meanwhile, David Davis has said he will not back down over the size of the UK’s divorce bill with Brussels.

The third round of talks ended on Thursday, with EU chief Brexit negotiator Michel Barnier saying there had been no “decisive” progress and attacking the UK Government for its lack of effort.

But Davis told the BBC’s Andrew Marr Show: “They have set this up to try to create pressure on us on money, that’s what it’s about, they are trying to play time against money.”

Davis’s team say much of the the money Brussels expect the UK to pay, some for aid projects, is a moral rather than legal obligation.

Comparing Brussels’ demands to a hotel bill presented to a guest, Davis said: “We are going through it line by line and they are finding it difficult because we have got good lawyers.”

He said Barnier “wants to put pressure on us, which is why the stance this week in the press conference – bluntly, I think it looked a bit silly because there plainly were things that we had achieved”.

Davis insisted he was not branding Barnier personally “silly”, adding: “I said the commission would make itself look silly.”

The Brexit Secretary dismissed as “nonsense” claims that the UK would pay a £50bn fee to exit the EU.

The “strict position” was that there was “no enforceable” legal basis for the UK to pay money to Brussels but “we are a country that meets its international obligations – but they have got to be there”.

Those obligations “may not be legal ones, they may be moral ones or political ones”, he added.

Brussels has long insisted it will only begin trade when there is “sufficient progress” on the money due, the rights of EU citizens living in the UK and UK citizens in the EU, and the border in Ireland.

Davis said money was “the thing that frightens them most” and insisted the UK would not be forced into backing down in order to begin trade talks in October.

“I’m not going to allow them to use the time pressure on that to somehow force us into doing X or Y or Z,” he said.

MPs head back to Westminster tomorrow, with Brexit and the the European Union (Withdrawal) Bill high on the agenda this week.

May said it was time for parliament to “play its part”, insisting that the legislation, which will be debated on Thursday, would receive proper scrutiny.

She added: “For us to grasp the great prize ahead of us, that contribution must fit with our shared aim: to help Britain make a success of Brexit and become that great global country we know we can be.”

Ian Blackford, the SNP’s leader in Westminster, said the parliamentary arithmetic after the election in June meant May no longer has a majority for her “hard-right hard Brexit.”

He warned the Tories of trying to proceed, and called on the other Scottish MPs to stand against a “power grab”.

Blackford said: ‘’Westminster is now a parliament of minorities – there is simply no mandate for the kind of hard extreme Brexit still being pursued by the Tories. I see opportunities in the session about to start for like-minded people to seek to protect the interests of the people of Scotland as well as the people of the rest of the UK.

‘’It is essential that all Scottish MPs work together to ensure that there must not be any detriment to Holyrood and the other devolved administrations, as the Tories still seem set to attempt a power grab in areas such as agriculture, fisheries and the environment. Opposing this – and protecting Scotland’s place in the single market – will be the most important things for Scottish MPs of all parties to pursue.

“Labour need to commit to permanent membership of the single market instead of their half-hearted transitional plans and ditch their longer-term backing for an extreme Tory Brexit.”

Meanwhile, the Scottish Government has called on ministers in Whitehall to clarify the support available for the fish and seafood sector once the UK leaves the EU.

Scotland receives 46 per cent of the UK’s share of the European Maritime and Fisheries Fund and £81m has been allocated to help Scottish businesses over the 2014-2020 funding period, with a further £53m from the Scottish Government.