The latest package – believed to be a parcel bomb – was found at the National Return Letter Centre in Belfast, where mail which has not been delivered and has no return to sender address is retained by Royal Mail.

It is the fifth package to be intercepted.

Strathclyde Police believe the package originally entered the postal system at around the same time as the devices that are already under investigation.

Officers from the Police Service of Northern Ireland recovered the package and alerted the Scotland force.

A spokeswoman for Strathclyde Police said the latest find would be made safe and handed over to officers in Scotland for further investigation.

It is understood the latest package was not addressed to a particular individual.

Chief Superintendent Ruaraidh Nicolson, of Strathclyde Police, said: “This latest discovery will become part of the ongoing investigation. We’d like to stress that there is still no intelligence to suggest that these packages pose a threat to the wider public and we would urge people to remain calm.

“The person or people sending these packages are clearly determined to cause fear and alarm to the individuals or organisations concerned.

“This is an utterly despicable crime, committed by an individual or individuals who are prepared to put people in harm’s way simply to attract attention to their callous and cowardly actions.

“We are grateful to Royal Mail for their continued vigilance and co-operation and would stress that workers have been given the appropriate safety advice.”

Last week Strathclyde Police said the two packages sent to Lennon, and others to lawyer Paul McBride QC and former MSP Trish Godman, were “designed to cause real harm to the person who opened them”.

Two of the four parcels were intercepted at Royal Mail sorting offices in the west of Scotland last month, both addressed to the Celtic manager. The first one was found in Saltcoats, Ayrshire, on March 4 and the second was intercepted on March 26 in Kirkintilloch, East Dunbartonshire.

Two days later a parcel for Ms Godman, former deputy presiding officer of the Scottish Parliament, was delivered to her constituency office.

Lennon, 39, has endured threats and abuse throughout his career and was forced to retire from representing Northern Ireland in international football after claiming he had received death threats from a paramilitary group.

He was the victim of a street attack in the west end of Glasgow in 2008 and earlier this year received a package containing bullets.

Celtic players Niall McGinn and Paddy McCourt, both from Northern Ireland, were also sent bullets.

McBride is one of Scotland’s most-recognised QCs and is a well-known Celtic fan.

Godman, who has stepped down as an MSP, was deputy presiding officer of the last session of the Scottish Parliament as well as Labour MSP for West Renfrewshire.

On her final day as an MSP, she was pictured wearing a Celtic football top at Holyrood.