A group calling itself the IRA has claimed it was behind the five suspicious packages sent to addresses on the British mainland.
Devices designed to start a small fire were posted to Waterloo railway station, offices at Heathrow and London City Airports, and the University of Glasgow last week.
- Read more: Suspicious packages latest: Second package found at Glasgow University
- Read more: Police investigate three small explosive devices in London
On Tuesday the Metropolitan Police said: "The Metropolitan Police and Police Scotland are aware of the claim of responsibility for the devices that were received at three buildings in London and at the University of Glasgow on March 5 and 6.
"The claim was received on Monday March 11 by a media outlet in Northern Ireland using a recognised codeword.
"The claim was allegedly made on behalf of the 'IRA'."
Police have so far found four of five devices that the group claim to have sent, and have given "extensive advice" on suspect packages to businesses and armed forces personnel.