Travellers on both sides of the Channel are facing further disruption as a strike looks set to keep the Port of Calais closed until Thursday.
A Kent police spokesman, who said the force is "currently aware" of disruption to Eurotunnel services in both directions, said: "It is anticipated that due to industrial action in France the Port in Calais will remain closed until Thursday."
Travellers planning to use the Channel Tunnel are urged to check with the operator before starting their journey.
Ferry services to and from the Port of Dover in Kent are also being affected by the strike, which started at the northern French port at 2.20pm yesterday.
Migrants took advantage of a wildcat strike last week by trying to board UK-bound lorries held up in queues at Calais.
Migrant numbers close to the port have swollen to more than 3,000 since April.
British truckers have reported facing violence and intimidation, and fear being fined if migrants clamber aboard their trucks.
Some now take lengthy detours to avoid Calais.
MyFerrylink workers went on strike after Eurotunnel, which owns the ships, sold the cross channel service after a competition authority ruling to rival operator DFDS. It left up to 600 jobs, including 70 in Dover, under threat.
MyFerrylink, which runs 16 crossings from Dover to Calais, had been due to officially stop all services from one minute past midnight on July 2. No promises could be made that services will restart before then.
A MyFerrylink spokesman said: "The company is ceasing operation - that is the reason for the strike."
Customers with an existing booking are being urged to call 0844 24 82 100 to get a refund or transfer to a Eurotunnel crossing, if any are available.
As temperatures are expected to soar, police urged travellers to take precautions including keeping their vehicles ventilated to avoid drowsiness and ensuring that babies, children, the elderly and animals are not left alone in stationary cars.
Kent Police, who warned motorists of "significant disruption" to their journeys, said phase two of Operation Stack had been put in place on the M20 southbound.
Non-freight traffic is being diverted on to the A20 from junction eight (Hollingbourne) and rejoining the motorway at junction nine (Ashford West). This allows backed-up freight traffic to park on the coast-bound carriageway of the M20.
A spokesman said: "It is important to note that as a result of a build-up of heavy freight traffic on the M20, A20 and roads in and around Dover, junction 13 of the M20 is also currently closed in the direction of the Port of Dover. All non-freight traffic will need to follow diversions or seek alternative routes."
Any freight traffic heading towards Dover will need to join the Operation Stack queue. This is where parts of the M20 are used to put lorries travelling towards the continent in queues, to ease congestion across the road network. It is introduced as an emergency measure after consultation with other agencies.