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 are said to bear "hallmarks of Northern Ireland-related terrorism", according to a Downing Street spokesperson.
One of the suspicious packages was stamped with an RoI (Republic of Ireland) postmark.
The remaining packages are understood to have been sent from different locations across Britain.
Sources have said that the packages were "crude" in design but "could have caused injury to others".
The seven suspect packages were identified as containing small, crude, but potentially viable devices and have been dealt with by bomb disposal units.
"Guidance has been issued to staff at all military establishments and Royal Mail asking them to be extra vigilant and to look out for any suspect packages and the screening procedures for mail to armed forces careers offices is being reviewed.
Four parcels were discovered at army careers offices in Oxford, Brighton, Canterbury and the Queensmere shopping centre in Slough today, counter-terrorism police officers said.
One package was found in Hampshire, yesterday, while two packets were found on Tuesday at the armed forces careers offices in Berkshire and Kent, the South East Counter Terrorism Unit said.
No group has yet taken responsibility for the packages.