A 20-mile section of motorway in Hampshire could be closed and turned into a lorry park if there are severe delays at Dover or Calais in the event of a no-deal Brexit, Vince Cable has been told.

Portsmouth is preparing to act as a relief port to Dover if there are serious delays at Calais, but like the cliffside Kent town, it cannot accommodate queuing lorries.

The M3 contingency between Winchester and Basingstoke is one option being considered by a resilience forum in Hampshire involving local councils and emergency services.

The “worst-case scenario” was revealed to Cable, the leader of the Liberal Democrats, as he visited Portsmouth international port on Tuesday to learn of preparations for the UK’s departure from the EU.

“What I am very concerned about is that the government simply hasn’t thought through this no-deal Brexit that they talk about. It is a very amateurish, Dad’s Army approach,” he said.

“The problem here is that Portsmouth will have to take considerably more lorries. They have to get through quickly, there will be customs checks, there is nowhere at the moment to store the lorries; the government is taking no interest in demands that there is additional lorry space; there are plans to close the M3 to use it for storage – extreme solutions of that kind,” Cable told ITV Meridian.

Dave Hardcastle, an assistant chief constable speaking for the resilience forum, said the preferred option was Dunsbury Park, off the A3, but until the government engaged with the group, it had to look at all options.

“We are still actively seeking holding areas for vehicles, of which Dunsbury Park is the preferred option, and we are looking for engagement from the Department for Transport to make that happen,” he said.

“We are still investigating further holding areas. Our least-preferred option is stacking lorries on the strategic road network, and we continue to work with Highways England to look for the least disruptive area if that becomes necessary.”

Portsmouth port handles 500 lorries a day, while 10,000 go through Dover.

Mike Sellars, the port’s director, told an ITV reporter that the distance between the freight gates and the motorway network was 13 lorry lengths and there would be no option but to make lorries queue on the public road network.

He said plans to use nearby land had been jettisoned because the owner, the Ministry of Defence, had refused to release it for no-deal use.

“There is an awful lot of infrastructure that needs to be put in place, and the cost, and whether that will be in place by the end of March, is another question, and who is going to pay for it,” Sellars said.

If the motorway connection between the M3 and Portsmouth were used for queueing, it would cause major disruption, he added.

“If the M275 motorway is blocked with lorries, this is the main route into the city. It does impact on people getting in and out of the city. This is a naval port as well, you have the Royal Navy, people getting into work, the hospitals – it would very quickly impact on the whole of the Portsmouth city area,” Sellars said.

On Monday, the government tested an emergency traffic system for Dover in the event of a no-deal Brexit.

Drivers who took part in the rehearsal in Kent said they had been told that if the situation became critical, lorries planning to travel via Calais would be instructed to drive to Caen to make an alternative arrangement to take the ferry to Portsmouth.

David Martin, who transports frozen and chilled foods for Int Logistics, said: “We’ve been told that they might make us use Caen and Portsmouth, but that is going to cost a lot more money in terms of hours and petrol. If you do eight miles to the gallon and do this journey across the channel two to three times a week, it will all add up.”

Gerald Vernon-Jackson, the Lib Dem leader of Portsmouth city council, said: “We are going to get queues on the motorway and then the whole of the city clogs up.”

A Highways England spokesman said: “We currently have no plans to use any section of the M3 or other parts of the strategic road network to hold lorries should there be disruption at Portsmouth port, and we would have significant concerns with any suggested plan that proposed it.

“We continue to support Hampshire resilience forum, which leads on contingency planning across the county on how we can help with any extra traffic the port are expecting.”