London's Heathrow Terminal 4 reopens after hazardous materials incident

One of the four terminals at London's Heathrow Airport that was evacuated earlier on Monday over what authorities called a "possible hazardous materials incident" has now reopened after it was declared safe, the airport said in an update.

Heathrow - Europe's busiest airport - apologised for the disruption, saying on social media platform X that it was "doing everything we can" to ensure all flights depart as planned.

Separately, a spokesperson for the London Fire Brigade said that it was standing down its response to the incident.

The British airport's Terminal 4 - serving both European and long-haul routes - was shut temporarily on Monday evening as specialist crews from the brigade were deployed to carry out an assessment.

Paramedics treated 21 patients and one was taken to a hospital, the capital's ambulance service said on social media. The nature of the incident has not been disclosed.

Specialist officers from London's Metropolitan Police also attended the scene and conducted a thorough search of the area, the police force said via email, adding that no trace of any "adverse substance" was found.

Unverified video clips shared online showed dozens of passengers waiting outside the terminal. The departures section on Heathrow's website showed flights continuing to take off.

The entire airport was shut for nearly a day in March when a huge fire at a nearby electrical substation knocked out its power, disrupting flight schedules around the world and stranding thousands of passengers.