Irish police are helping counter-terror officers investigate three small explosive devices sent to major transport hubs in London.

The packages were sent to Waterloo rail station, London City Airport's headquarters and Heathrow Airport on Tuesday.

Police said they were all A4-sized white postal bags containing yellow Jiffy bags, with specialist officers finding them to be small improvised explosive devices.

Scotland Yard added the devices appeared capable of "igniting a small fire when opened". The Met Police Counter Terrorism Command has launched an investigation.

IEDs found at London transport hubs 11 show all IEDs found at London transport hubs 1/11 Police officers, including one wearing a forensic suit, in a cordoned off area at Waterloo station REUTERS 2/11 A police officer stands guard outside the cordon at Waterloo AFP/Getty Images 3/11 Police officers investigate after an explosive device was sent to Waterloo station REUTERS 4/11 Police officers secure the scene where an IED was found near Waterloo station REUTERS 5/11 Police officers, including one wearing a forensic suit, in a cordoned off area at Waterloo station REUTERS 6/11 Police officers secure a cordoned off area at Waterloo station REUTERS 7/11 A British Transport Police vehicle at Waterloo station PA 8/11 A British Transport Police vehicle at Waterloo PA 9/11 Police at Waterloo after three explosive devices were found by police at three different locations across London EPA 10/11 Police at Waterloo after three explosive devices were found by police at three different locations across London EPA 11/11 Police at Waterloo after three explosive devices were found by police at three different locations across London EPA 1/11 Police officers, including one wearing a forensic suit, in a cordoned off area at Waterloo station REUTERS 2/11 A police officer stands guard outside the cordon at Waterloo AFP/Getty Images 3/11 Police officers investigate after an explosive device was sent to Waterloo station REUTERS 4/11 Police officers secure the scene where an IED was found near Waterloo station REUTERS 5/11 Police officers, including one wearing a forensic suit, in a cordoned off area at Waterloo station REUTERS 6/11 Police officers secure a cordoned off area at Waterloo station REUTERS 7/11 A British Transport Police vehicle at Waterloo station PA 8/11 A British Transport Police vehicle at Waterloo PA 9/11 Police at Waterloo after three explosive devices were found by police at three different locations across London EPA 10/11 Police at Waterloo after three explosive devices were found by police at three different locations across London EPA 11/11 Police at Waterloo after three explosive devices were found by police at three different locations across London EPA

Following the discovery of the IEDs, British Transport Police have stepped up patrols and their presence at rail stations and the London Underground network.

Images shared by Sky News showed the remains of the package sent to Heathrow and the envelope for the explosive device which was sent to Waterloo.

Both have stamps featuring red hearts that are believed to originate from Ireland. Irish police confirmed officers are assisting the Met Police.

A statement from the Met Police force said: "The Met Police Counter Terrorism Command has launched an investigation after three suspicious packages were received at buildings in London today, Tuesday, 5 March.

"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.

"The Met Police Counter Terrorism Command is treating the incidents as a linked series and is keeping an open mind regarding motives."

Scotland Yard confirmed the device sent to the Compass Centre near Heathrow Airport was opened by staff, "causing the device to initiate".

"This resulted in part of the package burning," the force added in a statement.

"No one was injured. The building was evacuated as a precaution. Specialist officers attended and made the device safe. The building remains closed as enquiries continue."

Mayor of London Sadiq Khan tweeted: "Our thanks go to police, security, transport staff and all involved for their swift actions to keep our city safe."

Sean O’Callaghan, from British Transport Police, said: “Understandably the discovery of these items may cause concern, however I would like to offer some reassurance to those passengers intending to travel this evening.

“We will have a number of specially trained officers on duty to keep commuters safe and reassured. Officers will be highly visible on station concourses, on board trains as well as the London Underground network. If you have any concerns, please speak with us or indeed a member or rail staff."

The packages were found today near Heathrow Airport in Hounslow, near City airport in Newham and at Waterloo station.

The incidents occurred within two hours of each other, with the buildings evacuated and road closures and cordons put up at Waterloo.

The first package was discovered at 9.55am at The Compass Centre in Hounslow.

British Transport Police then responded to reports of a suspicious package at Waterloo station at 11.40am.

The Met Police responded to another package at City Aviation House at London's City Airport in Newham half an hour later.

The force said: “Staff were evacuated from the building. Specialist officers are at the scene. Enquiries are ongoing.”

The Docklands Light Railway was suspended to London City Airport and all road access to the airport was closed for around two hours, although flights were still landing and walking access was possible.

A spokesman for City airport said it is open and flights are operating as normal.

A Heathrow spokeswoman said: “Earlier today, police responded to reports of an incident in the Compass Centre. Police response teams and the emergency services attended quickly, enabling colleagues to evacuate safely without injury.

“Heathrow Airport remains operational, flights are not impacted and passengers are able to travel as normal. The police are treating this as a criminal act and we will be assisting with this investigation.”

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."

London Waterloo's owner, Network Rail, would not comment on the incident.