Police Scotland and the Metropolitan Police called for increased vigilance after a claim by a group calling itself the IRA that a fifth parcel bomb remains unaccounted for.

Following the discovery of four suspect devices in packages sent to a variety of targets including Glasgow University, the group claimed responsibility using a recognised code word, and said another device had also been posted.

Now experts have warned the public should be prepared for more attacks like the letter bombs found in England and Scotland, because that violent republican splinter groups in Ireland have never really gone away.

As police search for a possible fifth package, the group which has claimed responsibility called itself IRA and used a recognised code word to verify their claims with police, but its identity is disputed.

The statement issue by the Metropolitan Police puts its name in quotes, saying the claim was "allegedly made on behalf of the 'IRA'".

Others have questioned whether the group is the same as the Provisional IRA, which declared a ceasefire in support of the Good Friday Agreement in 1997, particularly because one of the bombs targeted Glasgow University.

Historically, the IRA has eschewed attacks on Scottish soil.

Professor Feargal Cochrane, Director of the Conflict Analysis Research Centre at the University of Kent, said the group was likely to be a dissident faction, but one which will be concerning police.

"As I understand it is not the Provisional IRA as such, but a splinter group who have assumed the mantle of PIRA," he said.

"They are mostly dissidents but there are some former PIRA people in its ranks. It is much smaller than old PIRA but has been around for several years and has killed before."

Factors including Brexit and the collapse of power sharing in Northern Ireland may be encouraging such developments, Prof Cochrane said.

"It is potentially gaining traction due to Brexit fears and lack of institutions in Northern Ireland. The main question is why the targets were chosen? There is no obvious reason other than opportunity perhaps - but that will be the focus for the Police Service of Northern Ireland and other forces at the moment."

Earlier this year police and security services were reported to be concerned at a possible resurgence of dissident republicanism, fuelled by anxieties over Brexit, and following a car bombing outside Derry's courthouse.

A number of splinter groups are causing alarm, including the so-called New IRA, who often style themselves as simply the 'IRA', as well as a number of other factions which reject the peace process.

The New IRA was created partly by former members of the Real IRA, and is the biggest dissident republican group, but there is also the Continuity IRA and groups such as the Irish Republican Movement and Arm na Poblacht.

Dr Eamonn O'Kane, Reader in Conflict Studies at the University of Wolverhampton, said: "The Provisional IRA, as was, has gone in a very different direction and given up the use of violence but there has long been a rump unhappy with the trajectory of republicanism.

"The dissidents have never really gone away and in recent years groups such as the so-called New IRA have been sporadically active."

The letter bombs, and particularly the targeting of army recruitment officers or stations are both in line with previous militant republican tactics, Dr O'Kane added.

As a result, the mailing of one device to Glasgow University was less surprising, he argued. "If the device was addressed to someone involved in army recruitment, that might carry more weight than the fact that it was in Scotland," he said.

While Police forces have warned people to remain on alert for a fifth device, the claim that another parcel has been sent, as well as the four which were found last week may not be true, he added: "The tactic of such groups was always to spread concern and disruption, with hoax devices and calls,a s well as actual devices. So there may or may not be a fifth device."

Yesterday in a joint statement Police Scotland and the Met reiterated security advice warning people to be on their guard, particularly armed forces personnel, and those working in the business and education sectors.

A spokeswoman for Police Scotland said the same advice had averted a potentially much more serious situation at Glasgow University. She added: "Police Scotland and the Met have both provided protective security advice to armed forces personnel, business and education sector on prevention and what to look out for.

"That advice led directly to the interception of the device sent to the University of Glasgow and the advice is being reiterated in light of this claim."

A spokesman for the Metropolitan Police declined to speculate on the whereabouts of a fifth device and said investigations were ongoing. He added: "Those claiming responsibility have indicated five devices were sent. At this time, only four devices have been recovered.

"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 are working closely together to share any information or intelligence that could assist their respective inquiries.

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