Eurotunnel has taken the “unprecedented” step of cancelling thousands of tickets after air-conditioning units failed on trains amid sizzling temperatures.
The firm made the decision to stop passengers from travelling on Friday if they were due to return on the same day or Saturday in a bid to ease long queues.
It comes as forecasters warned the UK's all-time heat record of 38.5C, recorded in August 2003, could be broken.
Passengers told of their anger after they faced delays of up to six-and-a-half hours amid highs of 35C on Thursday.
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Many resorted to spraying themselves with water as they were forced to wait on the tarmac in Folkestone for hours.
Eurotunnel took the "unprecedented decision to cancel daytrip/overnight tickets" for travel on Friday due to "extreme temperatures affecting the shuttle air conditioning".
The firm also warned that ferry companies "do not have availability to take any of our customers".
The English end of the Folkestone-Calais link has been worst affected, with travellers forced to queue for up to 90 minutes just to check in and a further three and a half hours for the next available slot, according to the operator.
Passengers were urged to stock up with drinking water before arrival as there were long queues to check in with no access to facilities.
It is one of the busiest weeks of the year for cross-Channel travel as the start of many school summer holidays leads to a surge in family trips.
A Eurotunnel spokesman said: "Due to the exceptional and prolonged temperatures, some of the air-conditioning units on board our shuttles are struggling to produce sufficient cool air.
"Our priority is customer well-being and safety so we are unable to board vehicles to these carriages."
Passengers were advised to take plenty of drinking water and find toilet facilities before arriving.
Sarah-Jane Morgan, a digital merchandising manager going on holiday to the Loire Valley with her husband, was delayed by four hours.
She said passengers were "getting very annoyed" as they waited in a holding where staff had "no information".
The 34-year-old said: "You could hear children crying probably due to the heat and being stuck, and lots of people with dogs were very worried."
When she finally boarded a train the temperature was 36C, which she described as "pretty horrific".
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Paul Johnson, managing director of a construction company in Dover, said people were "trying to find shade wherever they can" and were "crowding into the packed terminal building".
Additonal reporting by Press Association
