An evacuation of a Heathrow Airport terminal over a mystery chemical incident which left 21 people needing medical treatment was caused by ‘mass hysteria’.

A ‘possible hazardous materials incident’ saw Terminal 4 shut for around three hours shortly after 5pm on Monday, forcing crowds of people to gather outside with their luggage while ambulances and fire crews rushed to the scene.

Twenty-one people were assessed by paramedics at the airport, while one person was taken to hospital, amid fears of a chemical leak.

However, police confirmed that ‘no trace of any adverse substance was found’ at the airport, while emergency services stood down their response at around 8:30pm.

‘Around 20 people reported an injury. None were deemed to be life-threatening or life-changing. Enquiries are ongoing,’ a spokesperson for the Metropolitan Police said.

The incident was a ‘mass hysteria’ type event, with no hazardous materials found, a police source told the BBC.

They added that the incident could have ‘snowballed’.

The terminal was declared ‘safe to reopen’ shortly after 8pm.

Emergency crews could be seen putting on protective white suits before entering the terminal
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Emergency crews could be seen putting on protective white suits before entering the terminal

Crowds of people were seen gathering outside Terminal 4 as ambulances and fire crews rushed to the scene
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Crowds of people were seen gathering outside Terminal 4 as ambulances and fire crews rushed to the scene

A terminal at Heathrow Airport was evacuated as emergency services responded to an 'incident'
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A terminal at Heathrow Airport was evacuated as emergency services responded to an ‘incident’

A Heathrow Airport spokesperson said: ‘Emergency services have confirmed Terminal 4 is safe to reopen and we are doing everything we can to ensure all flights 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.’

The London Ambulance Service said: ‘We were called at 4.50pm to Terminal 4 at Heathrow Airport. We sent ambulance crews, our cycle response unit, an incident response officer, and our hazardous area response team (HART). We treated 21 patients before taking one to hospital.’

Earlier, the airport had said that firefighters were responding to a ‘possible hazardous materials incident’.

‘Specialist crews have been deployed to carry out an assessment of the scene, and Terminal 4 has been evacuated as a precaution whilst firefighters respond,’ the spokesperson added.

Those evacuated were given bottles of water by airport staff while foil blankets were handed out to keep people warm.

Emergency crews were seen putting on protective white suits before entering the terminal.

Passengers returned to Heathrow Terminal 4 after it was shut for three hours due to a 'mass hysteria' event
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Passengers returned to Heathrow Terminal 4 after it was shut for three hours due to a ‘mass hysteria’ event

The terminal was declared 'safe to reopen' shortly after 8pm, with passengers returning to the terminal
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The terminal was declared ‘safe to reopen’ shortly after 8pm, with passengers returning to the terminal

Crowds of people were seen gathering outside Terminal 4 after a 'possible hazardous materials incident'
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Crowds of people were seen gathering outside Terminal 4 after a ‘possible hazardous materials incident’

A Heathrow staff member told passengers that the airport terminal would be closing after an 'incident'
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A Heathrow staff member told passengers that the airport terminal would be closing after an ‘incident’

Crowds of people waited outside the airport after Heathrow confirmed there was a 'possible hazardous materials incident'
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Crowds of people waited outside the airport after Heathrow confirmed there was a ‘possible hazardous materials incident’

Staff handed out water and emergency foil blankets as passengers were stuck outside the airport for three hours
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Staff handed out water and emergency foil blankets as passengers were stuck outside the airport for three hours

Harry Smith, 22, told The Telegraph he was checking in for a Qatar Airways flight to Vietnam when the terminal was evacuated.

‘I was escorted out of the check-in part of the terminal.

‘We saw people with hazmat suits and masks.’

Other witnesses reported seeing privacy screens being put up as passengers were evacuated around 160 yards from the terminal.

All other terminals were operating as normal while Terminal 4 was closed.

National Rail said that trains were unable to stop at the terminal while emergency services were responding to the incident.

Heathrow is the UK’s busiest airport.

In March, the airport was shut after a devastating electrical fire which sparked worldwide travel chaos and days of disruption for hundreds of thousands of passengers.

It was forced to close after its main electrical substation exploded and was set alight less than two miles away in the west London suburb of Hayes.

The closure impacted 270,000 journeys and was caused by ‘an unprecedented set of circumstances’, Heathrow chairman Lord Deighton said.