Heathrow Airport Terminal 4 has reopened after emergency services said they responded to an incident.

London Fire Brigade has stood down its response after earlier saying it had been called to a "possible hazardous materials incident".

Pictures began to filter through of crowds of travellers stuck outside with their luggage. Heathrow Airport closed and evacuated the terminal's check in.

The Metropolitan Police said in a statement that "no trace of any adverse substance was found". They added that that some 20 people had "reported an injury" and were assessed by paramedics - none were found to have life-threatening or life-changing injuries.

All in all, the incident was over within three-and-a-half hours.

We're now bringing our live coverage to a close, you can read more on this incident in our story.

At 17:00 BST, emergency services were called to Heathrow Airport Terminal 4 following reports of "a potential hazardous materials incident".

Specialist crews were deployed and assessed the scene, London Fire Brigade said, adding that part of the airport was evacuated as a precaution.

A spokesperson at Heathrow Airport said Terminal 4's check in was closed and evacuated, and passengers were asked not to travel to the terminal.

National Rail then said trains were unable to call at Heathrow Terminal 4, in a post on X.

There was little sign of disruption to flights out of the airport, with no delays or cancellations reported at Terminal 4.

At just after 20:00, a spokesperson for Heathrow Airport said Terminal 4 was reopening, after confirming it was safe to do so.

A short while later, the Fire Brigade announced they were "standing down" their response.

The Metropolitan Police said in a statement that they found "no trace of any adverse substance", adding that "around twenty people reported an injury, none were deemed to be life-threatening or life-changing".

Since then, BBC News understands that the events at Heathrow Airport are a "mass hysteria" type event.

BBC News understands that the events at Heathrow Airport are a "mass hysteria" type event, according to a source at the Met Police, with no hazardous material found.

Twenty people were assessed by paramedics at the scene with all except one discharged.

One person has been taken to hospital and the emergency services response has been stood down.

The source said an incident may have "snowballed".

The circumstances are still under investigation.

We're now seeing a statement from Met Police who say "no trace of any adverse substance was found" following reports "a potential hazardous materials incident" at Heathrow Airport.

Police were called to the airport's Terminal 4 at 16:56 BST, and specialist officers attended the scene alongside the London Fire Brigade and London Ambulance Service.

The emergency services conducted a thorough search of the area.

In the statement, they add that "around twenty people reported an injury, none were deemed to be life-threatening or life-changing".

The London Fire Brigade (LFB) is "standing down" its response to what was earlier described as a "possible hazardous materials incident at Heathrow Airport."

It says that specialist crews were deployed to carry out an assessment of the scene, while the Terminal 4 check-in was "evacuated as a precaution".

Around 20 people have been assessed on scene by paramedics from the London Ambulance Service, LFB says.

The cause of the incident remains under investigation at this time, the statement from the fire service concludes.

We can now bring you a full statement from Heathrow Airport, after it was announced that Terminal 4 would be reopening.

"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," a spokesperson for the airport says.

"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."

We've just received word from a spokesperson for Heathrow Airport, who says that Terminal 4 is reopening.

The spokesperson says a full statement will be delivered soon - we'll bring that to you when we have it.

The Metropolitan Police says that a few people have been taken ill at Heathrow Airport, but no-one is in a life threatening condition.

It adds that no arrests have been made.

As a reminder, Terminal 4 has been evacuated. London's Fire Brigade saying it is responding to a "possible hazardous materials incident" there.

We're expecting a formal statement from the Met soon, and will bring you those details when we have them.

Image source, FlightFocus365

As we've been reporting, we're not seeing much disruption at Heathrow Airport as yet.

Video footage in the last half an hour from FlightFocus365 - which streams live footage of aircraft at the airport - has shown several flights taking off.

At 17:01 BST this evening, the London Fire Brigade (LFB) was called to Heathrow Airport. LFB said it was responding to a "possible hazardous materials incident".

Terminal 4 was evacuated, a spokesperson from the airport said, and passengers were seen waiting outside.

National Rail said trains had been unable to call at the Terminal 4 stop as emergency services were "dealing with an incident".

What we don't know is what the specific issue is, nor what caused it. It's also unclear what the knock-on effect on air traffic will be - some flights appear to be going ahead, while others in various terminals are showing delays.

There isn't much sign of disruption to flights out of Heathrow so far. Here's where things stand for next two hours:

Terminal 2: The 20:35 Air India flight to Delhi is delayed

Terminal 3: The 19:20 British Airways flight to Vienna is delayed

Terminal 4, where the incident is taking place: No delays or cancellations reported

Terminal 5: The 19:25 flight to Barcelona, the 20:15 flight to Rome, the 20:25 to Amman, the 20:40 to Milan, 20:55 to Aberdeen - all operated by British Airways - are delayed

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One man who spoke to the BBC sent in these pictures when he said he was evacuated from terminal 4.

"Trains are unable to call at Heathrow Terminal 4 due to the emergency services dealing with an incident," National Rail says in a post on X.

A quick scan of the Heathrow departures board, external shows most flights appear to be running as normal at the moment.

We'll continue to monitor for any updates.

A spokesperson from Heathrow Airport just just sent us this statement: "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."

It says all other terminals are operating as normal and it will provide more information "as soon as we can".

As we've been reporting, firefighters and police are at Heathrow Airport.

Part of the airport has been evacuated, though the London Fire Brigade has not indicated which areas are affected.

Passengers have been seen waiting outside terminal 4, however.

We've just had a statement from the London Fire Brigade which says: "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.

"The brigade was first called about the incident at 17:01, and crews from Feltham, Heathrow, Wembley and surrounding fire stations have been sent to the scene."

Firefighters are responding to a "possible hazardous materials incident" at Heathrow Airport.

We'll bring you more on this breaking story shortly.