British counter-terrorism police launched an investigation on Tuesday after three suspicious packages were received in buildings at London's Heathrow and City airports and Waterloo mainline railway station in the city centre.
BTP, the Met Police and Scotland Yard said it was too early to confirm whether the incidents were linked and that "enquiries continue".
"The station is open and train services are running".
An hour later another package was found at a building close to City Airport, in east London. The building has been evacuated as a precaution. Nobody was injured in the incident, and specialist officers attended to make the device safe.
A second device was discovered in a post room at the Waterloo station around 11:40 a.m., and the area was cordoned off.
They have been assessed by specialist explosives officers and confirmed to be viable devices. Train services are continuing to operate as normal.
Police were also called to a report of a suspicious package at Aviation House at London City Airport shortly after midday (11pm AEDT). Flights are not affected. "This resulted in part of the package burning". That package was not opened. The building has since opened.
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The airport's operation centre remains closed following the incident.
A spokesman for London City Airport said that Aviation House was a staff-only building about three minutes from the terminal and no flights or passengers were affected.
Key transport hubs in the British capital have been urged to be vigilant for other possible devices.
"No arrests have been made".
Its owner, Network Rail, would not comment on the incident.
Despite the cordon being in place, services are still thought to be running normally.