A hazardous materials incident at Heathrow Airport's Terminal 4 prompted a significant emergency response, leading to the evacuation of the terminal's departure area.

While the incident caused initial disruptions, including advising passengers not to travel to Terminal 4 and delaying some train services, most flights continued as scheduled, and other terminals remained unaffected.

Transport Secretary Heidi Alexander monitored the situation, and social media footage showed passengers waiting outside, some wrapped in foil blankets, as authorities responded.

The situation remains ongoing, with investigations continuing to determine the cause, and further updates are expected.

After over three hours of investigation, authorities found no trace of harmful substances, and the incident is now believed to be a case of collective panic without any material cause.

One person was hospitalized, and approximately 20 people reported injuries, though none were life-threatening; emergency services responded swiftly to treat those affected.

BBC News characterized the incident as likely a case of 'mass hysteria,' suggesting it was not based on a real threat.

Emergency services arrived quickly to assess and manage the situation, with passengers and staff receiving support as investigations continue.

Passengers and staff are being supported on site while authorities investigate the nature of the supposed hazardous materials involved.

The incident caused delays to more than a dozen flights, including Qatar Airways and Etihad, with some delays extending up to three hours, affecting passenger connections.

The London Fire Brigade announced they were standing down from their response as the situation was brought under control, and normal operations at Heathrow resumed.

This incident follows previous disruptions at Heathrow, including a major shutdown in March caused by an electrical substation fire, highlighting ongoing safety concerns.