A man has been arrested after a "potential hazardous materials incident" trigged an evacuation of Terminal 4.

Specialist officers attended the scene where 21 people required medical treatment following "mass hysteria".

Emergency services recovered a canister believed to hold CS spray during a search of the airport.

"It is thought that this substance caused a reaction to those within the airport," the Metropolitan Police said in a statement.

A 57-year-old man had been arrested on suspicion of possession of a firearm (CS spray) and causing a public nuisance.