Counter-terror police are investigating three packages containing explosives found at Heathrow Airport, and Waterloo station.
The force's Counter Terrorism Command is treating it as a "linked series" and "keeping an open mind" about motives, the BBC reported on Tuesday.
Irish Police are assisting the Met as the Heathrow package had Republic of Ireland stamps.
Heathrow's Compass Centre was evacuated after the package was reported to police.
The device set fire when staff opened the bag.
The police said: "The packages - all A4-sized white postal bags containing yellow Jiffy bags - have been assessed by specialist officers to be small improvised explosive devices.
"These devices, at this early stage of the investigation, appear capable of igniting an initially small fire when opened."
A Heathrow spokeswoman said the airport would assist the police investigation into the "criminal act".
The package had Republic of Ireland stamps on it, said the report.
The Gardaí confirmed it was assisting the Met investigation.
The Compass Centre, which is an office for Heathrow staff rather than part of the
passenger terminals, remains closed.
Two more packages were found in the capital during the next three hours.
An area of Waterloo station was cordoned off after a second package was discovered in the post room.
One worker among a group of staff outside the Network Rail office said he found the package.
Asked about the discovery, he said: "I'm sorry, I've been told I can't talk about it."
About 100 workers were evacuated from City Aviation House at City Airport in Newham after authorities were informed about a third package.
The second two packages were not opened and have since been "made safe", police said.
Flights were not affected but Docklands Light Railway trains did not stop at City Airport for about an hour during the investigation.
Transport Secretary Chris Grayling urged people to report "anything suspicious" to police, while London Mayor Sadiq Khan added: "Our thanks go to police, security, transport staff and all involved for their swift actions to keep our city safe."
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(This story has not been edited by Business Standard staff and is auto-generated from a syndicated feed.)