Holidaymakers waiting to cross the channel by shuttle have endured gridlock and hours of delays on one of the busiest travel days of the year after "migrant activity" caused disruptions on the French side of the tunnel.
Eurotunnel said people travelling to the continent from Folkestone faced a delay of an hour to check in and of 30 minutes at the terminal itself.
It has now resumed running its usual schedule of four trains per hour but has a backlog of passengers.
The operator was forced to suspend services last night because of what it called "migrant activity" in Calais, with reports that migrants were trying to break on to trains. The situation has since been resolved but delays continue.
Operation Stack, which closes the M20 in Kent coast-bound to cars between junctions 8 and 11 to hold waiting freight traffic, was started this morning and is causing delays on the approach to cross-channel services.
Motorists vented their frustrations as they waited in lengthy queues this morning, posting photographs online as they sat in lines of traffic.
Allie Edmonds tweeted: "Sat on the slip road to Eurotunnel for hour and a half! Only moved twice!", while Jonathan Collins wrote: "On M20 1 mile to tunnel exit. Grid lock."
Alex Francis added: "Edging a little bit closer to Eurotunnel thanks to some creative rat running. Sorry Kent villages."
Eurotunnel tweeted this afternoon: "Customers who are arriving late at our Folkestone terminal due to traffic delays will be offered the next available space at check-in."
It said it could not currently sell tickets to anyone who has not made a reservation, saying: "We sincerely apologise for the inconvenience this will cause to your journey."
Kent Police has warned that Operation Stack, which holds freight traffic during disruptions to cross-channel services, could remain in place until at least tomorrow.
The Port of Dover remains open but it was advising passengers of traffic problems caused by Operation Stack.
Freight transport chiefs said Britain's freight industry is losing £750,000 a day because of the huge problems lorry drivers have faced this summer trying to cross the channel.
Philip Gomm, of the RAC Foundation, said: "Kent is a major gateway to and from Europe but once again that gate is well and truly shut. Operation Stack has become the rule rather than the exception, and on one of the busiest travel weekends of the year it is causing mayhem for hauliers, holidaymakers and local residents alike."
He added: "Solving the immigration crisis is clearly a job for heads of government but that doesn't mean officials here can't do more to keep South East England moving. The depressing thing is that cross-channel disruption is nothing new and Operation Stack has been with us for almost 30 years. Yet only now are we seriously considering how to address its shortcomings."
Issues with migrants entering the Eurotunnel terminal at Coquelles have been a nightly occurrence in recent weeks.
There were delays to the service on Thursday when the body of a suspected migrant was found on the roof of a Eurotunnel train at the Chunnel terminal in Folkestone.
The news came as video footage emerged apparently showing migrants travelling to Britain on empty freight train carriages.
An estimated 5,000 migrants displaced from countries including Syria, Libya and Eritrea are now believed to be camped in and around Calais.