Counter terrorist police have launched an investigation after suspicious packages containing traces of explosive devices were found near three major London transport hubs.
The A4-sized white postal bags containing yellow Jiffy bags were assessed by specialist officers who found small improvised explosive devices potentially capable of igniting an initially small fire when opened.
The Met Police Counter Terrorism Command is treating the incidents as a linked series and is keeping an open mind regarding motives.
Irish police also said they were assisting the Metropolitan Police with the investigation due to the stamps that were allegedly on the letters.
Police were called to a building near Heathrow Airport just before 10am on Tuesday after a package was ignited when it was opened by a member of staff.
No one was injured and the building was evacuated as a precaution.
Flights at nearby Heathrow Airport were not effected.
At just before noon, British Transport Police were called to reports of a suspicious package in the post room at London's Waterloo Station.
The package was not opened and specialist officers made the device safe.
A picture of the package sent to Heathrow airport was shared online by Sky News correspondent, Alistair Bunkall, who said the stamps "say Eire on. Rep of Ireland."
One of the images showed a partially-burned package addressed to the airport, which Mr Bunkall said had caught fire.
Cordons were in place at a small area outside the station on Cab Road, but train services continued to operate as normal.
Less than an hour later, a third security incident was sparked after police responded to a report of a suspicious package at offices at City Aviation House, Royal Docks, Newham near London City Airport.
Staff were evacuated from the building as a precaution.
DLR services to London City Airport were also suspended but are now fully operational. Flights to and from the airport were not affected.
London City Airport said in a statement: "We can confirm that City Aviation House has reopened and staff have returned safely, following the evacuation of approximately 100 staff at 12.15pm due to the discovery of a small improvised explosive device.
"Specialist police officers dealt with the device, which did not affect flight operations. The airport terminal remained open throughout. Inquiries by the Metropolitan Police are ongoing."
Aviation House is where a number of airport staff work and is a two-minute walk from the passenger terminal.
The building was reopened shortly before 4pm.
In a statement Heathrow Airport said "police are treating this as a criminal act and we will be assisting with this investigation".
In a post on Twitter, Mayor of London Sadiq Khan thanked police and transport staff.