A crash between two vehicles on the airfield injured two members of staff.
According to our information, a British Airways van and a airfield marshal car collided.
One man, in his 40s, has been taken to a West London hospital and his condition is unknown.
Another man was treated at the scene for non-life threatening injuries.
UPDATE 11:36 UTC The man has sadly passed in hospital, he suffered a cardiac arrest. The other man involved in the accident suffered a shoulder injury.
More than 20 flights have been delayed this morning, mostly from Terminal 5, with some held back for more than an hour.
Hundreds of people have been stuck on their planes or disembarked.
Watch Heathrow live on cam.airlive.net/lhr