London’s counter-terrorism police said they had launched an investigation into who mailed three small bombs to two airports and a major rail station on earlier this week.
No one was injured by the devices, one of which caused a small fire in an office building at Heathrow airport.
The Metropolitan Police received the first report of a suspicious device at Heathrow at 0955 GMT after staff opened a package which caught fire.
Later, a similar package was identified in the post room of London’s busiest rail station, Waterloo, and a third was found in an office at London City airport in east London.
Flights were unaffected, though a light rail line linking London City with central London was temporarily suspended.