The port of Dover has warned there will be serious traffic congestion once a week in the town and on surrounding routes unless the government achieves a Brexit deal involving frictionless trade.

Richard Christian, the port’s head of policy, said there would be “regular gridlock” in Kent in the event of a hard Brexit, and disruption to freight traffic on ferries and Eurotunnel services would have a profound impact on Britain’s economy.

“Operation Stack may be needed around once a week,” he said, referring to the police traffic control implemented in the summer of 2015, when industrial action in Calais led to lorries being parked on the M20 for 21 days.

His comments come as Jean-Paul Mulot, the permanent representative to the UK of Hauts-de-France, the region in which Calais is situated, told a Freight Transport Association conference in London: “The migrant crisis helped us to understand that it was really easy to have traffic jams on both sides of the Channel.”

“We can talk and talk and write lots of papers, but in the end, I’m not sure we are going to get to a suitable solution unless the authorities on both sides of the border are allowed to exchange ideas and plan for [a] post-Brexit future.

“The reality on the ground is very different to the big picture in Brussels. We keep saying endlessly we need help. Don’t think too big, know more of the ground and let’s try to engage.”

The government has said it will introduce a successor to Operation Stack, called Operation Brock, next year in the event of Brexit-related traffic problems.

Port chiefs said a two-minute delay in Dover would lead to a 17-mile queue of lorries on the M20. Under Operation Brock, one carriageway would be closed to traffic to provide temporary lorry parking.

Christian told the conference: “If the cross-Channel system falls, our collective way of life falls.”

Customs agents dealing with non-EU freight told the conference preparations by the government to make Dover ready for Brexit were “woefully inadequate”.

John James, the chairman of the largest customs clearance agency in the UK, said the government was unprepared for the consequences of leaving the customs union and single market.

The big problem was not random customs checks, but the clearance documentation required for every consignment, he added.

James said a new system for third-country trade being introduced by HMRC in January required 84 data fields to be filled for customs purposes – 34 more than the current system.

Each form takes 10-15 minutes to fill out and there was no sign of HMRC recruiting staff in Dover or training them, he added.

Before the single market was established in 1993, there were 300 customs officers; there are now 24 in east Kent, James said.

There were also previously 185 customs clearance agents doing the paperwork. “Today, there are only 17, and only five of them of any real size operating a 24-hours-a-day service,” he said.

“In 1993, there were between 2m and 2.5m entries; post-Brexit, there will be somewhere in excess of 25m, this including Dover and Eurotunnel. It is obvious to everyone that customs clearance will be woefully inadequate.”

There would also be queues at ports in the Netherlands, Belgium and France, he said, and if congestion were severe, ferry companies and Eurotunnel would have to reduce the frequency of their services.