Heathrow Airport’s Terminal 4 was evacuated while emergency services responded to what firefighters described as a «possible hazardous materials incident».
An airport spokesperson said at around 6.40pm that the check-in area of the terminal had been closed and evacuated while emergency services responded to an incident.
The spokesperson advised passengers not to travel to the terminal in west London. All other terminals at the UK’s busiest airport operated as normal.
The airport was declared «safe to reopen» shortly after 8pm.
In a statement, an airport spokesperson said: «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.
«We are very sorry for the disruption caused, the safety and security of our passengers and colleagues is our number one priority.
«We encourage passengers to check with their airline for the latest information about their flight this evening and our colleagues will be on hand into the night to assist.»
A London Fire Brigade (LFB) spokesperson initially described the incident as a «possible hazardous materials incident» and said that specialist crews had been deployed to carry out an assessment of the scene.
The spokesperson added: «The brigade was first called about the incident at 5.01pm, and crews from Feltham, Heathrow, Wembley and surrounding fire stations have been sent to the scene.»
The LFB said around 20 people had been assessed at the scene by paramedics from the London Ambulance Service.
It added that the cause of the incident remained under investigation.
A Metropolitan Police spokesperson said 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,» a spokesperson for the force said.
«Around twenty people reported an injury. None were deemed to be life-threatening or life-changing. Enquiries are ongoing.»
Eyewitness Paul Scott said he had just checked in and was about to go through security when the evacuation happened.
He told Sky News that everyone who was in the departure hall had been evacuated and passengers were ushered away to about 150 metres from the terminal building.
He said privacy screens had been put up and he saw a number of people in white suits arriving at the scene.
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Mr Scott said a man with a loudspeaker had said it was a specialist team.
He said the mood was fine, but it was a little confusing as no one really knew what was going on.
Mr Scott, who was planning to fly to Sri Lanka on business as part of a group of six, said people have been handing out foil blankets as the temperature was dropping.