Mystery surrounds an unknown illness which has spread though a passenger jet and forced it to turn round.
Up to seven of 13 crew members and "many passengers" were struck down with the strange disease - but at this stage no one knows what it is.
A British passenger has described panic on board the American Airlines flight from London Heathrow to LAX a when crew asked if there was a doctor on board.
A spokesman for the airline said there was "medical emergency" and people on the flight were "ill" but had no other information.
Flight AA109 was close to Keflavik, a town in southwest Iceland, when crew declared a medical emergency and decided to head back to London.
Passengers were asked if there was a doctor on board before the decision to abort the flight was taken at approximately 2.50pm.
Paramedics and fire crews surrounded the plane as it landed at Heathrow moments ago.
Lee Gunn, who was on board the flight, told the Mirror Online: "About 2.5 hours into the flight just as we were passing Iceland we had a Tannoy announcement asking for any doctors, nurses or medical professionals on board to report to the boarding doors to assist with unwell passengers.
"The lights then came on in the cabin and there was lots of commotion.
"I'm a bit of a plane geek myself so thought 'here we go, Reykjavik here we come' but it was announced about 20 minutes after we were going to divert back to LHR.
"It was also reported that 7 of the crew - 13 on board in total I believe - had fallen ill, along with 'many' passengers."
The flight was met by Paramedics as it touched down at Heathrow just moments ago - five hours after it first took off.
It is thought the passengers luggage may have been retained for checks by the carrier.
A statement from American Airlines said: "American Airlines Flight AA 109, a Boeing 777-300, operating to Los Angeles International Airport (LAX) from London Heathrow, is returning to Heathrow due to a medical emergency.
"The aircraft departed London Heathrow at 1205 local time and is expected to land at Heathrow at 1700 local time.
"We apologise to our customers for the inconvenience to their travel plans."
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