Counter-terrorism police are investigating three small explosive devices at London Waterloo station, Heathrow and City Airport.
The suspect packages – all A4-sized white postal bags containing yellow jiffy bags – are being treated as linked and specialists say they are capable of starting a “small fire when opened”.
Buildings at Heathrow and City have been evacuated but flights at both airports have not been affected.
The device at City Airport has been “made safe”, and the building reopened and rail services to the airport have returned to normal.
Train services at Waterloo station continue to run as normal, but a small area outside the station has been cordoned off.
'Evacuated as a precaution'
The cordon outside London Waterloo station.
In a statement about Heathrow, police said: “At approximately 09:55hrs the Met Police received a report of a suspicious package at The Compass Centre, Nelson Road, Hounslow.
“The package was opened by staff at the building, causing the device to initiate. This resulted in part of the package burning.
“No one was injured. The building was evacuated as a precaution. Specialist officers attended and made the device safe.
“The building remains closed as enquiries continue. The building is not based at Heathrow Airport, and flights have not been affected by this incident.”
They added: “The Met Police has issued advice to transport hubs across London to be vigilant for and report suspicious packages to police.
“No arrests have been made. Enquiries are on-going.”