Marks & Spencer has warned of a range of problems for food imports to the UK when Brexit rules change next month.
In a letter to suppliers, it said UK and EU bodies were not ready for new paperwork needed next month when a grace period on import checks ends.
It said in some EU states, offices were not open at weekends, which would cause "significant disruption" to imports.
Supply chains in the UK are already under stress due to shortages of lorry drivers and other staff.
The new rules mean lorry drivers importing goods will need some 700 pages of documents.
In a letter to suppliers, first reported by the Times, M&S set out a wide range of problems, including a lack of vets for essential checks, governments' unpreparedness - in some cases having not translated the rules into local languages, while it says some authorities "do not appear to know what will be required".
It also said suppliers had expressed concern that in some EU states, officials who issue Export Health Certificates - needed for trade in animal products - only work standard office hours from Monday to Friday.
As "modern food systems rely on importing food seven days a week... this working pattern will cause significant disruption to that import schedule and exacerbate the HGV driver shortage".
M&S says EU markets represent over 25% of all UK food imports, adding "If we don't see a more common sense approach to compliance, this is going to hurt everyone involved".
As well as EU member countries importing into Great Britain, the issue will markedly affect goods crossing from Great Britain into Northern Ireland.
Currently, supermarkets which send products to Northern Ireland from Great Britain face only light-touch checks, under a so-called "grace period" which delayed some of the new post-Brexit processes.
The Northern Ireland Protocol helps prevent the need for checks on the island of Ireland's internal border.
M&S says unless matters are simplified, there is a "real danger of disruption and delay at the EU to GB border that will lead to significant food waste across the sector, reductions in range and availability, and inflationary pressures".
Although reports have emerged that the UK government is expected to announce further delays to some of the Irish Sea border checks this week.
M&S warned in July it had already cut Christmas products in Northern Ireland due to concerns over the forthcoming post-Brexit customs checks.
Its chairman, former Conservative party MP, Archie Norman, said once light-touch export checks end, there would be "gaps on the shelves" there.