Police Scotland and the Metropolitan Police say suspect packages found in London and on the University of Glasgow campus last week are claimed to have been sent by the IRA.
Investigators say the claim on behalf of the IRA was received on Monday by a media outlet in Northern Ireland outlet using a recognised codeword.
A joint statement from the forces said: “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.
“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.”
Police have urged the public to remain vigilant and report anything suspicious via a confidential phone line (0800 789 321), filling in a secure form at www.gov.uk/act, or in an emergency dialling 999.
Last Wednesday, buildings at the University of Glasgow were evacuated and surrounding streets cordoned off after a suspicious package was received.
Specialist officers carried out a controlled explosion of the device.
It followed the discovery of three packages in London the previous day.
One found at The Compass Centre in Hounslow – near Heathrow Airport – was opened, causing the device to initiate and part of the package burning.
Later that morning, officers made safe a device discovered at Waterloo Station’s post room before doing the same at a location near London City Airport.
No injuries were caused by any of the devices in London or Glasgow.
Detectives from the Met’s Counter Terrorism Command are leading the investigation into the three packages received in London, whilst Police Scotland, under direction from the Crown, is leading the investigation into the package received at the University of Glasgow.
Both investigation teams say they are working closely together to share any information or intelligence that could assist their respective inquiries.
Enquiries continue and no arrests have been made at this time.