Heathrow Airport's Terminal 4 reopened to passengers shortly after 8 p.m. BST on Sunday after a precautionary evacuation prompted by reports of possible hazardous materials, the Metropolitan Police said.
The London Fire Brigade was called to the departure hall at about 5 p.m. BST and carried out an assessment of the scene, leading airport staff to evacuate the terminal's check-in area as a precaution. The Metropolitan Police later said no trace of any adverse substance had been found.
About 20 people were assessed by the London Ambulance Service; none was reported to be in a life-threatening or life-changing condition. A Heathrow spokesperson apologised for the disruption and said the airport was working with airlines to ensure flights departed on time. Flight-tracking data indicated disruption to landings and departures at Terminal 4 was minimal while the evacuation was under way.
A source at the Metropolitan Police told the BBC the reaction to the incident was in part down to "mass hysteria." The force confirmed officers attended the scene alongside fire and ambulance crews as part of the multi-agency response.
Hundreds of passengers were seen outside the terminal while emergency services dealt with the incident. Airport staff and emergency crews managed passenger flow and assisted those requiring medical assessment.
Heathrow stressed the evacuation was a precautionary measure taken to ensure passenger safety while crews investigated the report. The London Fire Brigade and other emergency services carried out checks before the airport announced the terminal was safe to reopen.
The incident is one of several recent security and safety alerts at major transport hubs that have prompted rapid multi-agency responses. Authorities at Heathrow and the emergency services did not report any ongoing threat after the assessment was completed and flights resumed normal operations from Terminal 4.
Airlines and airport staff worked to minimise disruption for travellers affected by the temporary evacuation. Further details about the initial cause of the report were not released by the Metropolitan Police, which said its investigation had found no evidence of hazardous material at the scene.