The paramilitary organisation are said to have claimed responsibility in a note to a media outlet in Northern Ireland using a codeword.
The group indicated five devices were sent but only four have been recovered, cops said today.
Counter-terror cops launched an investigation after the packages were discovered at Heathrow, London City Airport and Waterloo station on March 5.
A suspect package also led to the University of Glasgow being evacuated the next day with Scottish cops teaming up with the Metropolitan Police.
All the bombs were posted in A4-sized postal packages containing yellow Jiffy bags and were capable of igniting when opened.
They had a circular heart with a "love from Ireland" stamp on them – sparking fears the IRA were behind them.
It comes as the University of Suffolk was today evacuated after a suspicious package was found in the post room – but it is not known if it's linked to the previous incidents.
COPS PROBING CLAIMS
The Met Police said today: "The claim was received on Monday 11 March by a media outlet in Northern Ireland outlet using a recognised codeword.
"The claim was allegedly made on behalf of the ‘IRA’.
"The investigations into these devices continue and relevant enquiries are being made in relation to the claim that has been made.
The IRA
The Met hasn't specified which branch of the IRA has apparently claimed responsibility.
The IRA was disbanded after the decommissioning and has been inactive for several years but several groups have sprung up under the name.
The New IRA is a violent paramilitary group based in Republican districts of west Belfast with some activity in Dublin.
It is formed of dissident Republicans who reject Northern Ireland’s peace process, which saw paramilitary groups decommission weapons in order to end The Troubles.
In 2014, the New IRA sent failed devices to Army career offices in England and carried out a car bombing in Londonderry.
The New IRA was also behind an attack in 2016 when a prison officer died after a bomb exploded under his van in Belfast.
MI5’s threat warning of a Republican terror attack remains at "moderate" but police in Northern Ireland and Ireland fear a return to a hard border after Brexit, complete with customs and other checks, could be a target for militant groups.
"Given the packages received last week bore similarities to devices sent in the past which were linked to dissident groups associated with Northern Ireland-related terrorism, officers were already looking at this as a line of enquiry.
"However, we continue to keep an open mind and enquiries continue.
"We are also aware that those claiming responsibility have indicated five devices were sent. At this time, only four devices have been recovered."
They added no arrests have been no arrests.
'IRA' FEARS
Security chiefs said last week they suspected the devices were a warning from IRA renegades ahead of Brexit and the possibility of a hard Irish border.
They bore the hallmarks of Provisional IRA firebombs planted in London stores in the early 1990s, which sparked blazes causing thousands of pounds worth of damage.
The official threat of a UK mainland terror attack by dissident Republicans was lowered from substantial to moderate last year.
But there are fears of a backlash if a hard border between the Republic and Northern Ireland is re-established.
If the group is found to be responsible for the packages, it will be the first Republican explosives found on the mainland since 2014.
THREE PACKAGES REPORTED IN LESS THAN THREE HOURS
One of the packages exploded into flames at Heathrow Airport's Compass Centre – which houses the airport's head office – at around 9.55am last week.
No-one was injured but the building was evacuated as a precaution and a terror investigation was launched.
Officers were then called to reports of a suspicious package in the post room of Waterloo Station at around 11.40am.
The package had not been opened and specialist officers were called in to make the device safe.
A third report of a suspicious package was then received from London City Airport at around 12.10pm.
The package was not opened and no-one was injured but staff were evacuated as a precaution.
More to follow…
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