A counter-terror investigation has been launched after three small explosive devices were found at buildings at major transport hubs in London.

Here's what we know so far. 

Police responded to reports of suspicious packages at Waterloo train station and office buildings at Heathrow and London City airports around midday.

READ MORE: Explosives found at Heathrow Airport, London City Airport and Waterloo station 

Specialist officers were called to Waterloo Station, Heathrow Airport and London City Airport in separate incidents between 09.55am and 12.10pm on Tuesday.

Police Counter Terrorism Command said was being treated as a "linked series". Scotland Yard said officers are treating the incidents as linked and are keeping an open mind regarding motives.

All the packages were A4-sized white postal bags containing yellow Jiffy bags and appeared capable of igniting a small fire when opened. 

No one has been injured in the three incidents, police said.

It is not thought that anyone has been arrested. 

As a precaution, DLR services to London City Airport were suspended but are now fully operational. Flights to and from the airport have not been affected.

Those who see anything suspicious are being urged to report it to the police by calling 0800 789 321 or visit https://act.campaign.gov.uk.