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 co-ordinator for counter-terrorism policing, said that no link has 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.

Security sources suggested the packages' Irish insignia may have been a "concerted attempt" to make them appear as though they were posted from Ireland, but could not rule out that they had been.

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

He said: "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."

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 identify if there are more and then hopefully we can intercept them at an early stage."

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

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".=

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