Salad leaves and citrus fruits could be missing from supermarket shelves as a result of restrictions on UK-France trade, Sainsbury’s has warned.

The supermarket giant said France’s ban on freight hauliers from the UK could affect food supplies but assured customers that crucial Christmas dinner supplies are available and already in the country.

Transport Secretary Grant Shapps said the ban on accompanied freight is “slightly surprising”.

Thousands of lorries that were meant to travel across the English Channel on Monday have been told to stay away from Kent ports.

HGVs turning up at Dover on Monday morning were greeted with signs saying “French borders closed”, and were being turned away.

It came as the south-east of England grapples with a new variant of coronavirus that could be up to 70% more transmissible than the original strain.

A Sainsbury’s spokesman said: “All products for the Great British Christmas lunch are already in the country and we have plenty of these.

“If nothing changes, we will start to see gaps over the coming days on lettuce, some salad leaves, cauliflowers, broccoli and citrus fruit – all of which are imported from the Continent at this time of year.”

Mr Shapps said that “probably about 20%” of goods going into and out of the country passes through Kent.

He told Sky News: “But it’s not the mainstay. Most goods actually come in and out by unaccompanied containers and those will continue to flow.”

Asked what the shortages could be, Mr Shapps said: “Obviously we don’t want these links to be closed for too long, but it’s not unusual for them to be closed and disrupted.

“In the short term it’s not a specific problem. But of course the key is to get it resolved.”

While unaccompanied freight is exempt from the 48-hour ban, goods that would usually be transported on lorries driven on to ferries by drivers face being unable to cross the Channel to France.

This will mean that the vast majority of trade out of the port of Dover could be held up at the border.

Alex Veitch, general manager at Logistics UK – formerly known as the Freight Transport Association – said he is “genuinely not worried” about food shortages, and urged people not to panic-buy.

He told the PA news agency: “But at the same time, this does need sorting out because after a while there will potentially be fewer trucks coming this way (from Europe) because the haulage firms that provide that service will be very worried about their driver being stranded on the UK side.”

Items imported by lorry into the UK from Europe tend to be those with a “short shelf life”, he added.

Mr Veitch said UK export firms are also likely to be “pulling their hair out” as they rush to deliver goods to mainland Europe ahead of December 31 – the end of the post-Brexit transition period.

He said: “They are probably trying to do that in haste just in case there is no free trade agreement.

“If they are selling to an EU company, there is a pretty high chance at the moment that there won’t be a free trade deal, so their buyer might have to pay either very high or in some cases low tariffs on their products, depending what it is.”

Peter Denby, managing director of Lincoln-based haulage firm Denby Transport, said the ban could affect deliveries he had hoped to complete before the end of the transition period.

You cannot believe how many more bits of paper work and admin we are going to have to do post-January 1, compared to today

He told PA: “We still have some exports to ship that we had planned between Christmas and the new year.

“If we are unable to ship those, that then becomes a lot more complicated as far as our customers are concerned because they will have to provide export transit documentation and a whole raft of things

“You cannot believe how many more bits of paper work and admin we are going to have to do post-January 1, compared to today.”

Mr Denby said he had seven vehicles on the Continent, which makes up almost three-quarters of his fleet.

He said: “Not being able to ship for 48 hours is a bit of a problem, although we have probably dropped a bit lucky because it is Christmas, so we didn’t have anyone scheduled to go out on Sunday or Monday.”

In 2019, more than 1,235,000 self-propelled vehicles left the UK through the Kent port, according to Government statistics.

This equated to more than 97% of outbound movements through Dover.

Meanwhile, unaccompanied road units such as trailers as well as units such as containers that are lifted on to ferries accounted for less than 3%.