Photos showing crowds waiting outside the terminal with their luggage have been posted online as travellers search for answers. One man said: “Fire alarm activate at Heathrow term 4, all operations stopped. Crowds clueless.” The Airport tweeted: “We apologise if you have any delays to your journey. We would like to assure you that our team are working on this as we speak and we are looking to have this resolved promptly.
“Thank you for your patience.”
The evacuation comes as police at Gatwick Airport boarded a plane after a “security threat” was directed towards staff.
More to follow.