British Airways pilots were struck down by food poisoning after an India stopover.
One officer was 'incapacitated' and put on oxygen at 30,000 feet on the 10-and-a-half-hour flight to Heathrow.
The other two pilots managed to continue working, but their symptoms worsened after the plane landed in Britain.
The pilots had breakfast together in a local hotel lounge and were also found to have drunk bottled water supplied by an outside vendor.
Medical staff are collecting and analysing samples to determine the exact cause of the food poisoning.
The pilots had shared breakfast and drinks in the hotel lounge, and all drank bottled water provided by an external company.
They all filed Air Safety Reports and were asked to provide samples so doctors could try to pinpoint the source.
British Airways said passenger safety was not affected and that the flight operated safely.
Insiders at pilots' union Balpa reportedly said officials are angered because there had been 'dozens' of previous complaints about hygiene at their £130-a-night hotel in Hyderabad.
BA is now assessing its hotel options in the southern Indian city of Hyderabad.
The airline said: 'The welfare of our colleagues is a top priority and we are reviewing crew accommodation in this location following some isolated reports of sickness.
'During the flight one of our pilots became unwell.
'In line with established procedures, the aircraft continued safely to its destination and landed normally.
'The safety of our customers and crew is always our highest priority and at no point was the safety of the flight compromised.'
Read more