Two of the seven suspected explosive devices sent to British army recruitment offices this week are believed to have been sent from the island of Ireland.
The devices were found at locations in southeast England over the past week.
British police said they were "basic but viable" devices which posed a "low-level threat".
One of the suspicious packages was stamped with a Republic of Ireland postmark.
The British government has said the packages bore "hallmarks of Northern Ireland-related terrorism".
It is believed dissident republicans opposed to the peace process are behind the packages.
British Prime Minister David Cameron has chaired a meeting of the government's Cobra emergencies committee to discuss the suspicious parcels, which sources said were "crude" in design but "could have caused injury to others".