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, two of whom have been taken to hospital.
The 448 passengers on the Virgin charter flight were returning from a cruise in the Caribbean, MSC Cruises said.
A spokesman for the cruise company said: "We are still investigating what may have caused the illness and we are currently waiting for further updates from Gatwick Airport medical services.
"What we do know is that on MSC Preziosa, no cases of acute gastroenteritis have been reported in the past 14 days."
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.
MSC Cruises said it sent four staff members to Gatwick to provide further assistance to all passengers.
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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."