Counter-terror police are still working to identify a motive or suspect over the explosive packages sent to major transport hubs as sorting offices are on high alert for further devices.

Deputy Assistant Commissioner Dean Haydon, the senior national coordinator for counter-terrorism policing, said on Wednesday that no link had been made with Irish dissidents at this stage.

The packages that arrived at Waterloo railway station and offices at Heathrow and London City Airports on Tuesday were posted with Irish stamps and had Dublin as the return address, prompting Irish police to join the investigation.

But Mr Haydon said no message appeared to be contained within the packages, no sender had been identified, and no group had claimed responsibility.

“We are talking to our Irish counterparts but at the moment there’s nothing to indicate motivation of the sender or ideology, so I cannot confirm at the moment if it’s connected to any Ireland-related terrorist groups,” he said.

Asked whether there could be more packages, he replied: “They were sent through the postal system so we are not ruling that out.

We’ve only seen three but, as a result, we’ve sent our detective security messaging across the country to key places and locations that have sorting offices with a view to identifying if there are more and then hopefully we can intercept them at an early stage.

“But there’s no indication there’s any more.”

Scotland Yard said there was it was not investigating any other suspicious packages confirming there was no link to an evacuation at the University of Glasgow because of a suspect package in its mailroom.

The force also released images of the packages which have been circulated to transport workers and postal sorting staff across the country.

Commander Clarke Jarrett, head of the Met’s Police Counter Terrorism Command, said: “We have issued extensive advice to transport hubs and mail sorting companies to be vigilant for and report suspicious packages to police, as always we encourage anyone who sees something suspicious to report it.”

In a bid to identify a suspect, Mr Haydon said, forensic teams are scouring the packages for DNA and fingerprints in an effort to identify the sender.

He stressed they are small devices “not designed to kill”, but said they show “some degree of sophistication” that would require a certain level of capability to produce.

All 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”.

The senders’ addresses were given as Dublin, with the one addressed to Waterloo appearing to add Bus Eireann.

Detectives are treating the incidents as linked and are keeping an open mind regarding motives.

No-one was injured in the three incidents and no arrests have been made.

The first report of a suspicious package came from The Compass Centre, a building near Heathrow Airport’s boundary, shortly before 10am.

Scotland Yard said the package was opened by staff, causing the device to ignite and partially-burn the packaging.

British Transport Police were later called to a suspicious package in the post room at Waterloo station at 11.40am.

The package was not opened and the device was secured.

Shortly after midday, police were also called to a report of a suspicious package at Aviation House at London City Airport.

The package was not opened, the building was evacuated and specialist officers made the device safe. The building reopened.

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