The Road Haulage Association (RHA) made the claim today after the deployment of Operation Brock, which sees lorries heading for Europe driving at 30mph along the coastbound carriage of the M20.
All other traffic, including lorries carrying out UK deliveries, must now use a 50mph contraflow of two lanes in each direction on the London-bound side of the road.
Highways England deployed Operation Brock from 6am on Monday between junction eight for Maidstone and junction nine for Ashford.
It is a £25 million temporary no-Brexit plan to avoid traffic jams if large numbers of lorries descend on ports including Dover and experience delays crossing the English Channel.
An RHA spokeswoman said: “Highways England originally planned to implement Operation Brock as a Brexit contingency.
“The RHA is therefore mystified as to why, considering the fact that Brexit is now over two weeks away, Brock has already been activated.
“Kent motorists recently had to contend with a multitude of roadworks on the M2 and M20, often with full closures of both routes, some simultaneously.
“We think Highways England’s approach is inflexible and needs to change to take account of the prevailing situation. As things stand, Kent businesses and residents have to adapt their working patterns to accommodate Highways England plans. This is an outdated concept and must change.”
Highways England said the speed limits would be in place “indefinitely” with the contraflow until further notice while the country waits to hear when it will leave the EU and if this will be with or without a deal, a spokesman confirmed.
The success of Operation Brock will be monitored over coming days.
There have been no reports of major problems or disruption, according to Highways England.
But some drivers complained on Twitter about unclear and confusing signage and lack of information about access to the motorway from junction eight.
Highways England said there was access to the services and off at junction eight for lorries, cars and local traffic.
But the coastbound slip road is closed at this junction so any vehicles driving south need to go back to junction seven and around the roundabout, as the contraflow begins before junction 8.
The RHA said: “It’s vital that the tens of thousands of trucks and other vehicles that use this route every day get an indication as to how long this current ‘temporary’ situation is due to last.
“We’ve heard reports of insufficient signage up to junction 8 of the M20. Other reports of disruption include delays on the southbound A249 at Detling Hill heading towards junction seven of the M20.
“It’s certainly a comprehensive contingency plan but we’re very concerned that it’s already causing disruption.”
The RHA thinks a no-deal Brexit would be a “disaster for businesses”.
The spokeswoman said: “The infrastructure is not in place to cope with the frictional movements and businesses are just not ready.”
Dover MP Charlie Elphicke said he had “serious concerns” about introducing Operation Brock as it was “pointless during normal conditions”.
He also said using the airfield was a “bad idea” as it was too far from the port and along single-carriage road, adding: “That’s why this project is just kicking the can down the motorway. I want to see proper investment - in lorry parks and a dualled A2.”
Heidi Skinner, of the Tunbridge Wells-based Freight Transport Association, said it was too soon to tell whether the plan would work, adding: “We will just have to live with Brock until we come to a more permanent solution.”
Brock is a modification of Operation Stack, which could still be used at the same time to queue lorries along the M20.
There was chaos in 2015 after strikes and migrant activity in Calais led to major delays and Stack was brought in, closing the M20 in both directions for three weeks.
This cost the county around £1.45 million a day and the country’s economy an estimated £250 million a day, according to Kent County Council.
Stack was used again for the first time in four years earlier this month when Storm Gareth caused disruption at the Port of Dover.
Operation Brock includes plans to turn Manston Airport into a giant lorry park in the event of a no-deal Brexit.
The disused airfield near Ramsgate could become a giant HGV holding bay to prevent traffic jams around Channel ferry ports.
In January, a test of the site was branded pointless and a “farce” after just 89 lorries took part but officials insisted the numbers were sufficient to test the logistics of the plans.
The site could hold up to 4,000 lorries if needed.