The motorway from Dover is already so congested that the whole rush hour would have to be re-planned and more re-schedule, it is feared.

Fears for the “60-mile-per-day population of Kent” if customs officials were forced to return to land have been called into question following reports of endless delays at the Kent border.

A Department for Transport (DfT) spokesperson confirmed it is aware of plans to run an extra 630ft stretch of motorway between Trafalgar and Folkestone, twice the usual length of 90km, to “further strengthen” the already-stressed area.

They said no decision had been taken on whether the extra lanes could be opened in the south of the country.

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But some local campaigners have warned that a new border, implemented after Brexit, could mean “tremendous, terrible and unnecessary” queues for drivers unable to get on to the busy M20 and the Eastern Freeway, the Mail reports.

The increases are being mooted in light of Britain leaving the European Union without a backstop with Norway, which has a similar mechanism to avert any border disruption.

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Mr Thurman said it would mean queues of seven or eight lorries a day, similar to what happened between 2006 and 2010, when 55,000 roadworks and closures were done on both levels to create a new border.

“That would be at least 10 to 12 gridlocks every day, if not larger. It would be either extremely congested or very dangerous.

“This shouldn’t be treated as just another ‘complex’ British project – they should be dealing with it on the same scale as every other project in Britain,” he said.

“It is also, I think, quite critical that they do not open the border because then they would cause huge difficulties for Kent and the rest of the UK.

“The one shining light in this is that if border schemes are to be rolled out, they have to be assured that they will not make traffic the biggest barrier to moving across the Channel.”

In December 2017, the Conservatives committed to introducing a planning system that would provide “appropriate prioritisation” for controversial projects like the idea of a new Dover-to-Paris rail link, despite warnings from former ministers.

Mr Thurman believes further works to the southern border need to be prepared to ensure it is not “bogged down by” the delays that have long marred efforts to clear up Britain’s post-Brexit borders.

The director of policy at the charity ID Awareness, David Ellwood, said: “We should not be expecting that the problems of the past 30 years are going to disappear tomorrow.

“We have got to start planning for what is coming and the status quo is unlikely to happen now, but what we do have to do is look at the non-blocking issues at the other ends of the border, such as HGV permits, paperless immigration control and the like.

“I think it would be very foolish, in terms of the attention we are paying to this, to expect it will go away, even if there is a backstop agreed.”