Families are facing a five hour delay at the start of their summer holidays due to "prolonged and unprecedented temperatures" affecting the air conditioning on board Eurotunnel shuttles.
On Twitter, the cross-Channel service warned people not to travel "unless absolutely necessary".
Passengers stuck in queues have been tweeting about their frustration at the delays. Sarah Dodwell said she had been queueing on the M20:
Rebecca Kelly left Lewes in Sussex at 7am and made it to the Eurotunnel check in at 10:30am.
A spokesperson for Eurotunnel said: "Waiting time due to restricted shuttle capacity caused by the extreme and prolonged high temperatures.
"Ticket sales for travel today currently unavailable.
"The prolonged and unprecedented temperatures in the South East of England are affecting the air conditioning on board our shuttles. Due to the high level of traffic booked, we are currently unable to check-in any customers arriving more than two hours before their booked crossing time.
"Our teams are working hard to improve the situation as quickly as possible. Additional staff on hand to provide information.
"We strongly recommend that you stock up with water and take a comfort break prior to arriving at our Folkestone Terminal."
Meanwhile, travellers heading for the Port of Dover were told to expect long queues on the surrounding roads.
Travellers have been advised to allow an extra 90 minutes for security checks due to large queues at check-in, while passengers were advised to leave early when heading to the terminal due to congestion on the roads.
Gatwick Airport also warned of delays, writing on Twitter: “Adverse weather across Europe is affecting flights in and outbound from Gatwick Airport, this may result in delays and cancellations.
“Please contact your airline for specific flight information before you travel to the airport.”
These passengers have been telling us about their nightmare journeys so far.
Raj Baksi left Brighton at 7am and Sarah Dodwell and her family are unlikely to reach their holiday cottage in France until 9:30pm: