The claims were lobbied at the airline by trade union Unite, which represents 20,000 cabin crew members in the UK, following the diverting of a BA service to Heathrow last week after crew members became unwell.

Unite said it had been concerned by BA referring internally to the incident as an “odour event”.

The union has pushed for an inquiry into supposed illnesses caused by exposure to contaminated air in aircraft, otherwise known as “aerotoxic syndrome”, The Guardian reports.

At present the condition is not recognised by the Civil Aviation Authority (CAA), or BA.

The union believed its argument was strengthened after the BA service from San Francisco to London was  diverted to Vancouver on October 25, when 11 crew reported feeling sick after inhaling fumes in the cabin.

All 25 crew members were later taken to hospital upon landing as a precaution.

Communications between the aircraft and air traffic control reveal the pilot asked for permission to land due to a “fume event”, involving “toxic gas-like fumes”, The Guardian reports.

According to Unite, a similar incident occurred the following day on a BA service from Heathrow to Los Angeles, during which, the union claims, the flight continued as normal despite staff having to put on oxygen masks to avoid fumes.

Unite’s director of legal services, Howard Beckett, said: “It is clear from all the reports we’ve received and the exchanges between the flight deck and air traffic control that the incident onboard the diverted BA flight from San Francisco to London Heathrow was more serious than a mere ‘odour event’.

“Downplaying serious toxic fume events onboard aircraft as “odour events’ smacks of spin and an attempt to manipulate official statistics to downplay how widespread the problem really is in the industry.

“Fume events and continued exposure to contaminated cabin air can lead to serious ill health with long-term debilitating effects on people’s well-being.”

BA said the incidents had been reported fully by both crew and the airline itself and were later passed on to the CAA.

A BA spokesperson said: “Safety is always our priority. There has been no change in the way in which we investigate reports of this nature. We continue to conduct thorough and detailed investigations which we share with the CAA. We always encourage our people to report any potential incident to allow us to investigate them.”