For some, it is just a case of making sure they have ample supplies of their favourite marmalade, or at least the basics like “baked beans and bog roll”.

Other so-called “Brexit preppers”, however, seem ready to summon up images of no-deal ushering in something akin to the Apocalypse.

“Allow at least three litres of water per adult per day,” says one online prepper guide published last June. “You can line your toilet with plastic bags in the event of water being switched off and dispose of the contents away from your house.

“[For warmth] gather in one room and use extra blankets to line windows and doorways…”

Theresa May, of course, has said members of the public do not need to stockpile and should take “reassurance and comfort” from her government’s official preparations for the possibility of a no-deal Brexit.

So how much of this Brexit prepping is sensible, and how much is alarmist overreaction?

Should we be stockpiling food?

“The short answer is no,” says the British Retail Consortium (BRC).

This is despite the pretty chilling letter it sent to MPs on Monday warning – with the backing of supermarket chief executives – of the potential consequences of no-deal Brexit.

And despite the fact that nearly one third of food consumed in the UK comes over what, for now, is a frictionless border with other EU states.

For non-perishables like tins of beans, explained Andrew Opie, the consortium’s director of food and sustainability, “There is no need for consumers to buy more than normal, because stores have been increasing their supplies a little, just to make sure they are ready for the initial period after March 29.”

And for perishable goods like fresh fruit and vegetables, stockpiling – by anybody – is futile.

“It’s not practical or particularly safe for anyone to store them,” said Mr Opie. “If there was any obvious way in which governments or businesses could solve the fresh produce problem in a no-deal Brexit, we would do. The reality is, the solution is to avoid a no-deal Brexit.”

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With retailers typically storing no more than two weeks’ worth of stock, others have suggested further reasons why personal stockpiling might be unwise.

One food supply expert, speaking anonymously because of how sensitive Brexit has become, explained: “It’s like the fears of petrol shortages caused by the fuel protests a few years ago. People think there is a petrol shortage, so they all rush off and buy petrol, and that’s the only reason you end up with a shortage. It becomes a self-fulfilling prophecy.

“By everyone keeping calm, you can prevent the very situation you are worried about from occurring.”

Should we be stockpiling medicines?

The Royal College of GPs (RCGP) says no, but not in a very reassuring way.

Professor Helen Stokes-Lampard, Chair of the RCGPs issued her guidance to The Independent with some pretty striking caveats.

“It is not within our realm of expertise,” she said, “To prepare autonomously for a situation [no-deal Brexit] with so many unknowns, and we should not be expected to as there are over 7,000 GP surgeries in England alone.

“The governing Council of the RCGP recently passed a motion to oppose Brexit and to call for a ‘People’s Vote’ on the grounds that Brexit in any form could potentially damage the NHS and patient care in a number of ways that were not made clear at the time of the initial referendum.”

But having said that, she advised: “We do support the guidance that GPs should not stockpile medicines and devices at a practice-level as this has the potential to create or exacerbate shortages on a wider-scale.”

“We understand that patients may be concerned about their access to medicines and devices,” she concluded, “But would ask them not to take it upon themselves to stockpile - or put their prescribing healthcare professional in a difficult position by asking them to help them to do so.”

Instead, said Professor Stokes-Lampard, patients should “look to the government to reassure the public that these measures are unnecessary”.

As for how that government reassurance is going, in October England’s most senior department of health official, permanent secretary Sir Chris Wormald announced publicly that maintaining supplies of essential medicines would be “very complex” in the event of no-deal.

A few hours later, Theresa May insisted the department of health was working to ensure “we have plans in place” to keep medicines flowing, even in the absence of a deal.

Will there be food shortages and price rises in the event of a no-deal Brexit?

Almost certainly, according to the letter warning MPs about the consequences of no-deal that was sent by the BRC and the UK chief executives of: Sainsbury’s, Asda, Marks & Spencer, the Co-Op, Waitrose, Costcutter, Lidl, Pret A Manger, KFC and McDonald’s.

The letter sought to explain to MPs the disruption no-deal Brexit would cause to Britain’s just-in-time food system, where fresh, perishable produce pulled out of the ground in Spain can be on UK supermarket shelves the following day.

Nearly a third of all food eaten in the UK is EU-produced, and having a no-deal Brexit in March might be particularly bad timing because in spring a huge proportion of some perishables – 90 per cent of lettuces, 80 per cent of tomatoes, 70 per cent of soft fruit – comes from mainland Europe.

In this, Dover – as former Brexit secretary Dominic Raab belatedly realised - is particularly important: 38 per cent of all EU goods imports come through Dover and Folkestone.

And it has been estimated that if you delay lorries at Dover for just two extra minutes, it can eventually cause tailbacks of around 30 miles. (Hence the government’s attempt, in January, to simulate a 6,000-vehicle backlog at the Port of Dover with 150 lorries, which became 89 lorries after nearly half the drivers didn’t bother to show up.)

The letter stated that such delays can be expected in the event of no-deal.

MPs were warned: “Even if the UK government does not undertake checks on products at the border, there will still be major disruption at Calais as the French government has said it will enforce sanitary and customs checks on exports from the EU.

“Government data suggest freight trade between Calais and Dover may reduce by 87 per cent against current levels as a result. For consumers, this will reduce the availability and shelf life of many products in our stores.”

As for the supermarkets – as opposed to the customers – being able to further stockpile non-perishable items: “All frozen and chilled storage is already being used and there is very little general warehousing space available in the UK.”

The letter also warned that as well as there being supply difficulties, prices may be pushed up by the new WTO rules tariff regime that would follow a no-deal Brexit.

Currently, the letter explained, tariffs are paid on only around 10 per cent of food imports to the UK. MPs were told that proportion would go up if the UK reverted to WTO rules, which would “greatly increase import costs, which could in turn put upward pressure on food prices.”

And so, the letter from one retail consortium, seven major grocers, and three big food chains concluded: “We are extremely concerned that our customers will be among the first to experience the realities of a no-deal Brexit. We anticipate significant risks to maintaining the choice, quality and durability of food, and there will be inevitable pressure on prices.”

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Although when it comes to the more extreme prepper predictions, there seems to be no need to worry about the suggestion that “the UK would survive no longer than five days on internal resources in a no-deal situation.”

The Independent was told it was “remarkably unlikely” that Britain would ever have to rely completely on its own internal resources: trade would not cease altogether, it would just get more complicated.

We have been told that once we free ourselves from the shackles of Brussels, we will become a truly “global Britain”. Can’t we simply airlift in the necessary food from our many new trading partners around the world?

Not, it seems, without the risk of significant increases in food prices.

“If you have to airlift in a whole range of products,” said Mr Opie of the BRC, “It adds massively to costs.

“The risk is that retailers whose margins are already incredibly squeezed can’t absorb those costs. Either they go out of business or they pass the costs on to the consumer in the form of higher prices.”

There was, he added, another problem with ending reliance on lorry loads of EU food by air freighting everything in from all over the world. It might not be possible.

“We simply don’t have the capacity to air freight everything,” said Mr Opie.

“At periods of peak demand, like just before Christmas, we had 130 lorry loads of citrus imports alone coming through our ports every day. That’s just for one type of product. We can’t just switch everything to air freight.”

But no-deal Brexit won’t happen, so surely no harm will have come from all this worrying about it?

Just the contingency planning for no-deal is racking up massive costs, said Mr Opie.

“The costs are increasing by the day,” he said. “Every day, we are having to pump money into planning, in terms of people’s time, in terms of trying to find alternative supply routes and alternative supplies.