Heathrow Airport was evacuated in a “possible hazardous materials incident" as the result of “mass hysteria”, a police source has said.
Emergency services scrambled to Terminal 4 at Britain’s busiest airport in Hounslow, west London after around 20 people reported an injury.
However, Scotland Yard later confirmed that “no trace of any adverse substance was found” at the site that evening.
A Met Police source told BBC the incident was believed to be a "mass hysteria” type event and the incident may have "snowballed".
The terminal — which serves 14 million people each year — reopened to passengers shortly after 8pm, three hours after the London Fire Brigade carried out an initial assessment.
Crews from fire stations in Feltham, Heathrow, Wembley and the surrounding areas had attended the scene amid fers of a chemical leak.
Heathrow Airport Terminal 4 has reopened after a suspected hazardous material incident caused a temporary closure earlier this afternoon.
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At least 21 people were assessed by the London Ambulance Service for injuries, none of whom was deemed to be in a life-threatening or life-changing condition. One person was admitted to hospital.
Check-in at the terminal was initially suspended and passengers were evacuated as a precaution while police, firefighters and paramedics responded.
Heathrow apologised for the delays caused to people’s journeys.
“We are very sorry for the disruption caused, the safety and security of our passengers and colleagues is our number one priority,” the airport said in a statement.
Footage shared online showed travellers with their luggage waiting outside the airport, with some wrapped in foil blankets.
The safety of our passengers is our number one priority
A witness, named Paul Scott, told Sky News that he saw a number of people in white suits arriving at the terminal.
Airport chiefs had advised travellers not to travel to the terminal, while National Rail said trains were unable to call at the accompanying station.
The move caused further misery for commuters who were unable to reach Heathrow using the Piccadilly line because of strikes by the National Union of Rail, Maritime and Transport Workers (RMT) over working conditions.
According to the airport’s departures board, flights from Terminal 4 were largely unaffected.
A Met spokesman said: “On Monday, September 8, at 4.56pm, police were called to a potential hazardous materials incident at Heathrow Airport, Terminal 4.
“Specialist officers attended the scene alongside the London fire brigade and London Ambulance Service and conducted a thorough search of the area.
“No trace of any adverse substance was found. Around 20 people reported an injury. None were deemed to be life-threatening or life-changing.”
The cause of the incident remains under investigation at this time.
First opened in 1986 by King Charles, the then-Prince of Wales, and his then-wife Princess Diana, Terminal 4 was originally the base for British Airways before they moved to Terminal 5 in 2008.
It is now primarily used for a variety of long-haul flights by airlines such as KLM, Air France, Etihad, Qatar Airways and Malaysia Airlines.
