Police say the "IRA" has claimed responsibility for the four letterbombs that were received at three buildings in London and one in Glasgow earlier this month.

Scotland Yard said a media outlet in Northern Ireland received the claim of responsibility using a recognised codeword.

Meanwhile, an alert has also been issued over a possible fifth letterbomb, sent through the post to a British Army recruitment officer, which may not have been discovered yet.

A Met Police spokesman said on Tuesday: "The claim was allegedly made on behalf of the ‘IRA’.

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

"The investigations into these devices continue and relevant enquiries are being made in relation to the claim that has been made.

"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."

The Met also said those claiming responsibility indicated five explosive devices were sent - though only four have been recovered. It has been claimed a parcel was also sent to a British army recruitment officer.

They were found at office blocks near Heathrow Airport and London City Airport, as well as Waterloo station, on Tuesday last week. The device at Glasgow University was reported on Wednesday. No one was injured.

The force continued: "Extensive advice has already been issued to relevant businesses and sectors to be vigilant for and report suspicious packages to police.

"This advice was previously sent to armed forces personnel and is being reiterated again in light of this claim.

"We continue to urge the public to remain vigilant and report anything suspicious to police."

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

The stamps appeared to be those issued by the Irish postal service for Valentine’s Day 2018, featuring a heart motif and the words “Love Eire N”.

Terror detectives from the Met are continuing to investigate the three packages received in London, while Police Scotland is leading the probe into the device at Glasgow University.

No arrests have been made.