The captain made an emergency landing at Zurich Airport when the First Officer fell unconscious at the controls on Boxing Day.
The A320 Airbus had taken off from Heathrow and was an hour away from Athens when the co-pilot started feeling unwell.
The pilot had initially turned around and headed back to London but made an unscheduled landing in Switzerland when his colleague’s condition deteriorated.
The first officer was taken to hospital and has since been released but it is not yet known what caused him to fall unconscious.
A BA insider told MailOnline: ‘The First Officer had initially complained of feeling ill, but on the return to Heathrow passed out.
‘The Captain thought the situation was serious enough to divert rather than try and return to Heathrow.
‘It is nothing to do with COVID-19 and there are no reports of any toxic fumes in the cockpit which is a common cause of pilots suddenly becoming ill.’
The passenger plane spent five hours in Zurich before making its way to Athens.
A BA spokesman said: ‘The safety of our customers and crew is always our top priority and the flight continued to its destination as planned.’
The only thing worse than starting something and failing… is not starting something.