British Airways has suspended all flights to Cairo due to heightened security concerns.
BA said its flights to the Egyptian capital would be cancelled for seven days 'as a precaution,' however they did not specify the nature of the security alert.
However it was reported the decision may be due to fears over increased ISIS activity in Egypt.
Airline Lufthansa also suspended flights last night but they were resumed today.
Passengers told of their disappointment after one BA flight from Heathrow to Cairo was cancelled. A letter from the airline read: "We constantly review our security arrangements at all our airports around the world and have suspended flights to Cairo for seven days as a precaution to allow for further assessment."
Official Foreign Office advice, updated on Friday, states: "There's a heightened risk of terrorism against aviation. Additional security measures are in place for flights departing from Egypt to the UK."
Passenger Christine Shelbourne, 70, from Surrey told the BBC: "The check-in staff reissued my boarding pass and I tried again but that didn't work either and we were told to try again in half an hour.
"Whether they knew anything I don't know but my husband told me the flight had been cancelled before they did. There were no suggestions or help from staff about alternative flights.
"My 11-year-old grandson is heartbroken - he's been looking forward to the trip for months. We're just not going now,' she added. It was handled badly to be honest. My grandson is currently looking for flights for us - he's devastated."
One angry passenger tweeted: "Currently about to board a @British_Airways flight to Cairo and it's cancelled.
"Not for a day. Not for two days but for seven. Security risk. Someone knows something we don't."
One BA customer, Rasha, who asked to keep her surname private, said she was booked on a flight on July 25 but was alerted by text about the suspensions.
The IT worker, who is from Egypt and has lived with her family in Britain for six years, said she was supposed to meet her husband, children and other relatives.
"It's just terrible," she said. "It may seem like I'm just missing my flight for a holiday, but I really need to go."
British Airways said: "We constantly review our security arrangements at all our airports around the world, and have suspended flights to Cairo for seven days as a precaution to allow for further assessment.
"The safety and security of our customers and crew is always our priority, and we would never operate an aircraft unless it was safe to do so."
BA was also among several major carriers to announce in June they would reroute flights to avoid flying over the Strait of Hormuz.
The move, also taken by Australia's Qantas, Dutch carrier KLM, Emirates, Germany's Lufthansa, Malaysia Airlines and Singapore Airlines, followed the downing of a US drone by Iran.