A 10-mile-long motorway in south-east England can be shut and turned into a lorry park to cope with tailbacks from the port of Dover if Britain crashes out of the EU with no deal, under one plan being considered by the government.

The M26 in Kent would become a “holding area” for up to 1,400 goods vehicles to try to ease the gridlock as up to 10,000 lorries a day suddenly require customs checks to enter the EU.

The idea is just one part of a vast effort by the government to brace Britain for the possibility of chaos if relations sour with the EU and co-operation ends after next March.

With London and Brussels still deeply at odds over their post-Brexit relationship, the chances of leaving without a deal are growing rapidly and contingency planning is being stepped up on both sides.

But after 45 years of integration, a sudden rupture would affect almost every part of British life, with companies already stockpiling food and medicine, and the government looking at whether it needs to use the military to keep the country running.

Weekly advice on no-deal planning

John Manzoni, the chief executive of the civil service, told MPs on Thursday that a breakdown between the UK and the EU could have some “horrendous consequences” that the UK is not yet ready to cope with.

“There needs to be a narrative to say, ‘Actually we don’t want this to happen but we have to prepare just in case it does and here are all the things that we have to do’,” he said. “There are supply chains for food and medicines; we have to put in place contingencies for those.”

Over the summer, the government will start issuing weekly advice to businesses and households on how to prepare for a “disorderly” Brexit and 70 technical notes are being prepared. As part of the preparations, some 250,000 small businesses will be asked to start making customs declarations, in a dry run for a hard Brexit.

The public campaign about no-deal planning is partly designed to show the EU that it should take Theresa May seriously when she says she is prepared to walk away without a deal. But the government also wants to convey the message that preparing for no deal is not just a matter for the civil service, but also one for companies and individuals.

Businesses already spending millions

UK businesses are already spending heavily on their preparations. Adam Marshall, head of the British Chambers of Commerce, said companies have been thinking through the potential impact of changes to cross-border trade, staffing, contracts, VAT and intellectual property.

“The government needs to do the same — and demonstrate to business that it has clear answers to all the big questions that are within its own control,” he said.

A recent survey by the CBI found that 61 per cent of members were developing a contingency plan for a “no deal” scenario: but the vast majority found this difficult because of the lack of information available.

In the pharmaceuticals sector, companies said they were aware of potential shortages of vital medicines.

David Jefferys, senior vice-president for Eisai, a Japanese pharmaceuticals company, said a total of 3,000 to 4,000 marketing authorisations for medicines are held in the UK and will need to be moved to mainland Europe.

His company is transferring licences for about 60 medicines to Germany “at considerable cost”. He said each transfer costs “about €7,200 [ . . .] just for them to change the name from Eisai Europe in Hatfield to Eisai GMBH in Frankfurt”.

“We will have to be stockpiling products on both sides of the channel to keep security of supply . . . We reckon that the whole enterprise is [costing us] between £5m and £10m,” Dr Jefferys added.

No deal plans being activated

George Bridges, a former Brexit minister who is now advising the chair of the bank Santander, also said that companies are running out of patience.

“Some people think we can strike a deal with the EU in, say, mid-February, and that will stop business from activating their no-deal plans, but that’s totally unrealistic,” he said. “They will start implementing them incrementally and then the question is how reversible some of that is if there is a deal.”

Gavin Darby, chief executive of Premier Foods, the Mr Kipling cake maker, who is also president of the Food and Drink Federation, the industry group, said some companies were increasing their stock of ingredients on UK soil.

“Smart companies will have worked out where the pinch points are with ingredients and be building up inventory,” he said but declined to comment on what Premier was doing.

Ian Wright, director-general of the Food and Drink Federation, said food manufacturers had two main concerns. “One is getting product in through ports around the Brexit date, so that you can continue to manufacture. The other concern is — what will happen to the price of ingredients?

“If there’s no deal, you’d expect sterling to tank, so people are trying to hedge against those concerns by storing ingredients like spices that can be kept in dry storage, as well as chilled and frozen food, although there is a shortage of chilled and frozen warehousing in the south of England.”

Peter Ward, chief executive of the UK Warehousing Association, said: “Half the food we consumer is imported; 80 per cent of that imported food comes from Europe and 90 per cent of that comes through the Dover corridor. If I were a property investor, I’d be scouring the M20 corridor now because that’s where they’ll be demand.”

Whitehall sees some successes

In Whitehall, preparations for no-deal, led by Tom Shinner, head of policy and delivery co-ordination at the Department for Exiting the European Union (Dexeu), have seen some important successes.

HM Revenue & Customs, for example, has said it is now close to being able to cope with a surge of users for its new Customs Declaration Service computer system if there is no deal.

In the face of heavy traffic at the ports, however, HMRC said on Thursday that it would prioritise keeping the “flow moving” over collecting duties. About 5,000 new customs officials will be needed if there is no Brexit deal, and 1,113 have so far been hired, HMRC said.

If there is no deal next March, MPs expect they will have to pass about 1,000 new statutory instruments through Parliament in a matter of days to bring regulations into UK law, including safety certificates for airlines, maintaining the enforceability of financial contracts and even keeping the Channel tunnel open.

“It would be like 2008 when Parliament had an endless sitting to deal with the crisis,” said one official. “At least we’ve had a recent experience of this.” Some MPs note that even without a full Brexit deal, it may be possible to strike bilateral agreements on aviation and nuclear regulation to mitigate the worst possible outcome. Details are alarming for public

The challenges stretch in all directions. The National Audit Office disclosed this week that the government does not yet have plans for a system to issue as many as 7m international driving licences to British tourists who wish to drive in the EU if the current arrangements are not honoured. The Department for Environment, Food and Rural Affairs is considering racehorse passports.

Philip Hammond, the chancellor, put aside £3bn for no-deal contingency planning in his November Budget and an increase in “no deal” activity has been evident across Whitehall in recent days.

But many aspects of the contingency planning — like the M26 idea and the use of RAF aircraft to ferry supplies of food and medicine around the country — are so drastic that they risk alarming the public.

“The M26 is a major artery and you have to pray it wouldn’t come to this,” said James Hookham, deputy chief executive of the Freight Transport Association.

“It’s certainly one of the options they are looking at in government. It would effectively mean that Cobra had taken over the road network as a matter of national security,” he added, referring to the government’s emergency council.