A British Airways engineer has died after an accident which meant hundreds of passengers were removed from planes on the tarmac.
Emergency teams have been rushed to the scene after the incident which happened around 6am at Terminal 5.
Hundreds of people have been stuck on their planes, and many have complained of confusion and delays.
There was a crash between two vehicles on the airfield, injuring two members of staff.
One man, in his 40s, was taken to a West London hospital and later died.
In a statement, Scotland Yard said: "Police were called at 6.06am on Wednesday, February 14, following a collision between two airport vehicles on the taxiway at Heathrow Airport.
"Officers attended with London Ambulance Service colleagues.
"One male is believed to have sustained a broken shoulder; his injuries are not life-threatening or life-changing.
"Another male, aged in his 40s, was taken to a west London hospital with serious injuries. He has since died.
"Officers have informed next of kin. We await formal identification.
"A post-mortem will be scheduled in due course.
"There have been no arrests.
"The Health and Safety Executive has been informed.
"Officers from the Met's Serious Collision Investigation Unit are investigating."
A Heathrow spokesperson said: “We can confirm that following a serious accident involving two vehicles on our airfield, an airport colleague has passed away. Our deepest condolences go to the family and friends affected by this accident. We will be fully cooperating with the Police in the investigation which will follow.”
A BA spokeswoman said: "We are deeply saddened by this tragic incident and are offering our full care and support to his family and colleagues."
More than 20 flights have been delayed this morning, mostly from Terminal 5, with some held back for more than an hour.
The airport confirmed the incident on Twitter and said it is unlikely to cause delays.
Hi Alaistair, following an incident on the airfield involving two members of staff, an investigation is underway and we are working with the Police. This incident didn't involve any passengers and is not expected to cause any impact on journeys today.— Heathrow Airport (@HeathrowAirport) February 14, 2018
However, passengers have complained that they have been delayed.
One Twitter user wrote: "Apparently there has been an accident on one of the runways at @HeathrowAirport and many planes (including this one) are parked on the apron with unspecified delays of 'hours'. No public news of this and no useful updates from @British_Airways — anyone out there know any more?"
Another said: "With respect, it is impacting journeys departing B gates - BA710 to Zurich delayed by +2 hrs and aircraft change required. Sending good wishes to injured staff."
And another wrote: "Well that's disingenuous when 200 people in this plane are stuck. Tosh."
One passenger who had been on a Zurich-bound flight, told The Sun: "We boarded and waited and then the captain gave a series of apologies and further delay announcements.
"It looks like some airport or airline staff may have been injured and police required an area to be closed off which prevented some planes from leaving their gates."
"The gate was surrounded by various airport service vehicles including two fire engines. There were blue lights everywhere."
British Airways confirmed: "We have apologised to customers for the delay to a small number of our flights this morning, following a serious incident involving two operational vehicles at the airport. We are doing all we can to ensure they will be able to depart as soon as possible. "