London's Heathrow Airport reopened on Monday evening, as no adverse substance was found following reports of a "possible hazardous materials incident."

"Emergency services have confirmed Terminal 4 is safe to reopen," the airport said on social platform X. "And we are doing everything we can to ensure all flights depart as planned today."

The Terminal 4 check-in was closed and evacuated earlier this evening after reports of incidents involving "possible hazardous materials." Emergency services have responded to the incident.

Video clips and photos shared online show that large numbers of people were gathering outside the terminal building, with water and blankets being handed out.

According to a statement from the Metropolitan Police, no trace of an adverse substance was found. It added that around 20 people reported injuries, but with no life-threatening conditions.

Heathrow Airport is the primary and largest international airport serving London. According to its official website, over 67 million passengers travel through the airport annually on services offered by 90 airlines, travelling to over 180 destinations in over 90 countries.