BREAKING.

Emergency services have responded to the scene, which Sky News has reported as a 'possible hazardous materials incident'.

Footage on social media shows numerous ambulances outside of Terminal Four, perhaps as a precaution.

A Heathrow spokesperson said: "Terminal 4 check-in has been closed and evacuated while emergency services respond to an incident.

"We are asking passengers not to travel to Terminal 4 and supporting those on site. All other terminals are operating as normal. We will provide further information as soon as we can."

A London Fire Brigade spokesperson said: "Firefighters are responding to a possible hazardous materials incident at Heathrow Airport.

"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."

National Rail has said that trains were not calling at Heathrow Terminal 4 due to 'emergency services dealing with an incident'.

More to follow.