Detectives are yet to release their findings on three packages containing explosives found at Heathrow Airport, London City Airport and Waterloo station. No one so far has claimed any responsibility.
The “small improvised explosive devices” were found in A4 postal bags, the Metropolitan Police said.
The force’s Counter Terrorism Command is treating it as a “linked series” and “keeping an open mind” about motives.
Irish police are assisting the Met as the Heathrow and Waterloo packages had Republic of Ireland stamps.
Heathrow’s Compass Centre was evacuated after a package was reported to police at about 09:55 GMT.
The device set on fire when staff opened the bag.
Scotland Yard 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 Gardaí confirmed it was also assisting the Met.
Ireland’s postal service identified the stamps as its “Love & Wedding” design for greeting cards, wedding invitations and thank-you cards.
The Compass Centre, which is an office for Heathrow staff rather than part of the passenger terminals, remains closed.