Heathrow's Terminal 4's check-in area was "closed and evacuated" Monday evening as emergency services responded to an "incident", the airport's operator said on social media.
The UK's main international airport, one of the world's busiest air hubs, told passengers not to travel to the terminal.
The London Fire Brigade said its firefighters were responding to a "possible hazardous materials incident", according to a statement reported by the BBC.
"Specialist crews have been deployed to carry out an assessment of the scene, and part of the airport has been evacuated as a precaution whilst firefighters respond," the fire brigade said.
The UK's National Rail network said in a statement on X that its trains were "unable to call" at the terminal, southwest of London, as a result.
Heathrow said that "all other terminals are operating as normal".
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