No trace of any “adverse substance” has been found after emergency services were called to Heathrow airport on Monday, the Metropolitan Police has said.

Terminal 4 was partially evacuated as fire crews investigated “possible hazardous materials” at the airport.

Heathrow’s spokesperson apologised for the disruption and said they were working with airlines to ensure flights departed on time.

The London Fire Brigade (LFB) said crews were called to the terminal at about 17:00 BST to carry out an assessment of the scene, and Terminal 4 check-in was evacuated as a precaution.

Earlier, National Rail said trains were unable to call at the terminal “due to the emergency services dealing with an incident”.

Firefighters were stood down at around 20:20 BST and both the LFB and Metropolitan Police have said the cause of the incident remains under investigation.

In a statement, the Met 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.”

The Met said none of the people who reported injuries were “deemed to be in a life-threatening or life-changing condition”.

“Enquiries are ongoing,” the force added.

Twenty-one people were assessed by the London Ambulance Service, with 20 discharged at the scene and one taken to hospital.

A Heathrow spokesperson said it reopened to passengers shortly after 20:00 and they were “very sorry for the disruption caused”.

In a post on X, the airport said it was doing “everything we can to ensure flights depart as planned today”.

Disruption to flights landing and departing from Terminal 4 appeared minimal while the evacuation was under way, according to flight data.