One side of the main motorway to the Port of Dover will be shut to non-freight traffic from Monday morning as part of testing for a possible no-deal Brexit.
The scheme, known as Operation Brock, is to ease congestion in Kent if traffic is brought to a standstill due to disruption caused if the UK leaves the EU without an agreement and there is no longer frictionless trade.
If there are delays at the Dover-Calais crossing after the UK quits the bloc, the government has said up to 11,000 lorries will be able to queue on the M20.
From Monday morning, two lanes of the coastbound carriageway between junction eight for Maidstone and junction nine at Ashford will be shut for non-freight traffic, in order to test the contraflow plans.
Lorries heading for mainland Europe will be routed down the coastbound carriageway, with a 30mph speed limit in place.
Cars and other vehicles will be directed onto the London-bound carriageway – with two lanes in each direction operating at 50mph.
Three lanes of traffic in each direction and a 50mph speed limit could be restored later in the week if Operation Brock is not immediately required.
Operation Brock will also allow lorries to be routed onto a disused air field and another motorway, the coastbound M26 if needed.