Heathrow terminal reopens after 'possible hazardous materials incident'

London: Heathrow's Terminal 4 "was safe to reopen", the airport operator said Monday evening a few hours after its check-in area was temporarily closed and evacuated over a "possible hazardous materials incident".

"Emergency services have confirmed Terminal 4 is safe to reopen and we are doing everything we can to ensure all flights to depart as planned today," the operator of the UK's main international airport said on social media.

The London Fire Brigade said it was standing down operations around three hours after it was called to respond to a "possible hazardous materials incident" at the terminal, part of one of the world's busiest air hubs.

"Around 20 people have been assessed on scene by paramedics from the London Ambulance Service," the fire brigade said.

London's Metropolitan Police force later issued a statement saying "no trace of any adverse substance was found."

"We are very sorry for the disruption caused, the safety and security of our passengers and colleagues is our number one priority," the airport added.

It said other terminals had been operating as normal throughout the incident.