A new safety system at the Dartford Crossing will turn around vehicles considered too unsafe to travel through the tunnel.
A traffic light and barrier system will stop traffic before it gets to the tunnel entrance.
It will come into force next month.
It will also enable dangerous goods vehicles to be safely escorted through the tunnels and stop vehicles entering the tunnels in the event of an incident.
Tunnel closures will be required overnight during the go-live weekend to introduce the system. To minimise the disruption, one tunnel will remain open at all times.