Police are investigating suspicious packages at Waterloo Station and City Airport following an earlier incident at Heathrow Airport which is now being investigated.
All the packages were A4-sized white postal bags containing yellow jiffy bags, and according to the police, appeared capable of igniting a small fire when opened.
The second call came in at approximately 11:40 a.m.
All transport stations throughout Britain have been advised to "be vigilant" and to report any suspicious packages to police.
A further package was found at Waterloo Station at around 11am, but was not opened.
This means the full weight of resources and expertise of one of the world's most experienced counter terrorism teams will be trying to get to the bottom of who sent the packages and why.
There were no injuries reported.
"Cordons are in place, however train services continue to operate as normal at this time".
Police arrived at Heathrow to deal with a suspicious package in an outbuilding at 9.55am and it has been made safe.
The first package went off at Heathrow Airport offices at 10am this morning.
Investigators said an employee opened one of the packages, causing it to burst into flames. Nobody was injured in the incidents and flights in and out of the airports were not affected.
The building has since been reopened.
The Compass Centre, which serves as Heathrow Airport Limited's head office, remains closed.
Its owner, Network Rail, would not comment on the incident.
United Kingdom police have launched a terror investigation after bombs were sent to three London-area transit hubs.
Regarding the London City Airport incident, the Metropolitan Police said: 'At approximately 12.10 today, Tuesday, 5 March, police were called to a report of a suspicious package at London City Airport Aviation House, Royal Docks.
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