Gardai are probing whether the New IRA was behind a number of explosive devices posted from Dublin to key transport hubs in London.
Officers are working on the theory that the plot was carried out to cause chaos ahead of the UK's planned withdrawal from the EU at the end of the month.
Counter-terrorist offi- cers in Dublin and London are now working closely to identify those responsible for the three devices.
Police were called yesterday to investigate suspicious packages at Waterloo train station and office buildings at Heathrow and London City airports .
Officers from the garda security and intelligence section in Phoenix Park are playing a crucial role in the investigation.
Their help was requested by the Metropolitan Police's counter-terrorist command because Dublin postmarks and An Post stamps were found on at least two of the letters.
One device exploded, causing little damage except to the package containing it, but the other two were kept intact.
Police said the devices, posted in A4-sized white postal packages containing yellow jiffy bags, were capable of igniting a small fire, when opened.
The stamps appeared similar to some issued by An Post for Valentine's Day 2018, featuring a heart motif and the words "Love Eire N".
Sky News correspondent Alistair Bunkall tweeted a picture which he said showed the package which was sent to Heathrow Airport. The partially-burned package appeared to have Dublin written in black ink in the bottom right corner of the envelope, which Mr Bunkall said had caught fire.
Another picture showed a jiffy bag addressed to Waterloo, with its sender's address appearing to be Bus Eireann, Dublin.
The operator said police had not been in touch, with a spokeswoman saying: "Bus Eireann are currently not aware of this and we have no further comment." The devices were later described as unsophisticated.
Police in London received the first report of a suspicious package at the Compass Centre, a building near Heathrow, shortly before 10am.
It exploded as it was being opened by staff but nobody was injured.
A statement from Scotland Yard said: "The package was opened by staff at the building, causing the device to initiate.
"This resulted in part of the package burning."
The other two packages, found in the post room at Waterloo at 11.40am and at Aviation House at London City Airport shortly after midday, were dealt with by specialist officers. Searches were reportedly going on to locate any other similar packages that might have been sent but had not yet been identified. Although there was no claim of responsibility for the incidents, gardai here and police in London have placed a dissident republican faction, likely to have been the New IRA, at the top of the suspect list.
Garda intelligence has been indicating for several months that the New IRA was preparing to ramp up its activities in 2019 and hoping to exploit any opportunities for terrorist action that might arise from Brexit, particularly if it resulted in a hard border.
Anti-terrorist officers in London said they were satisfied that the three incidents were linked and they were keeping an open mind on motives.
The New IRA has already carried out a car bomb attack outside a courthouse in Derry city in January. Last week, gardai discovered an arms dump in a wooded area outside Omeath, Co Louth, on the Cooley peninsula. It included two rifles and about a quarter kilo of Semtex plastic explosive, enough to act as a booster for three under-car bombs. This has been linked to a separate dissident faction.
Advice has been issued to transport hubs across London to be vigilant.
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."
A Metropolitan Police spokesman said: "At approximately 09:55, the Met Police received a report of a suspicious package at The Compass Centre, Nelson Road, Hounslow.
"The package was opened by staff at the building, causing the device to initiate.
"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."