Food rationing must be introduced immediately to prevent the UK running short of fresh fruit and vegetables, three experts have warned Boris Johnson.

The professors say supplies come predominantly from Spain and Italy, the two European countries hit hardest by the coronavirus pandemic.

Furthermore, the switch by supermarkets to “just-in-time” supply chains has increased the risk of severe shortages in the weeks and months to come.

“Thirty years ago, the UK’s food retailers carried 10-12 days of stock – now they have just 24-36 hours of stock,” Professor Erik Millstone, of the University of Sussex, told The Independent.

Tim Lang, a professor of food policy at the University of London, echoed the call, saying the situation facing the country is so serious it is “Brexit times 20”.

“We’ve got to do demand management, not just blame people when they start getting three days or a week’s food supply,” he said.

“That’s nothing. Nothing. In my childhood, that’s what everyone did.”

The professors – the third is Terry Marsden, of Cardiff University – have written to the prime minister, criticising his government’s response to stockpiling as “weak and unconvincing”.

They warn that a staggering 8.4 million people across the UK are at risk of potentially running short of the food they need – and food banks cannot cope with rising demand.

Supplies of fresh fruit, vegetables and salads from Spain and Italy will drop drastically if there are too few people to pick, pack and transport them – or if their government impose export restrictions.

The letter also warns that people in the UK are set to buy even more food at supermarkets, now that pubs, cafes and restaurants have been shut down.

The professors, who all have long experience in food policy research, urge the government to:

* Introduce rationing based on the ‘Eatwell Plate’, a recognised measure of the foods and drinks needed for a healthy balanced diet.

* Consider a national scheme of vouchers that can be redeemed for fruit and vegetables.

* Ensure that healthy food can and will be delivered to all those who self-isolate or are quarantined.

Prof Millstone, head of Sussex University’s science policy research unit, added: “Stock is all in trucks or on the shelves – in pursuit of ‘efficiency’ and cost-cutting, the system has lost its resilience.

“We need to rebuild resilience for our food system and reduce our reliance on imports and just-in-time deliveries. We must also ensure that the food we have is equitably distributed.”

George Eustice, the environment secretary, has argued there is no shortage of food in the country, with manufacturers having increased production by 50 per cent.

The retail industry has also insisted there is enough food for everyone – but customers have been faced with empty shelves and have struggled to get items.

A government spokesperson rejected talk of a looming crisis, saying: “Retailers are continuing to monitor their supply chains and taking all the necessary steps to ensure consumers have the food and supplies they need.