Passengers and crew on a Virgin Atlantic flight were put into quarantine due to widespread sickness on board the plane.
The aircraft was met by emergency services when it landed at Gatwick Airport about 05:25 following an eight-hour flight from Bridgetown in Barbados.
Passenger Trevor Wilson said “a bad chesty cough, possibly a chest infection” was affecting people.
The airline said it was investigating.
Thirty people were assessed and treated, three of which have been taken to hospital.
The company said the flight had been chartered by a cruise company and was not a scheduled Virgin flight. A spokesman said the problem had not originated on the plane.
Mr Wilson tweeted: “This flight was exclusively cruise passengers from the MSC Preziosa. The illness seems to have originated on board ship not the plane. Five members of cabin crew became sick on flight.”
A spokesman for the airport said emergency staff had taken the ill passengers to an assessment centre for treatment.
“The other passengers have now disembarked as normal,” he said.
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A Virgin Atlantic spokeswoman said: “A number of customers on board a charter flight from Barbados to London Gatwick this morning reported feeling unwell onboard.
“As a precaution the plane was met by the relevant authorities who made the decision to screen everyone onboard, in line with standard health and safety procedures
“We are working closely with London Gatwick Airport and medical teams to offer assistance to customers, and will conduct a full investigation into the circumstances.”
She said screening was a standard procedure when a health issue was reported on board a plane.
“There is no reason to believe this is anything other than an isolated incident.”