One of the suspicious packages was stamped with an RoI (Republic of Ireland) postmark but no group has claimed responsibility for the attack, sources said.
Prime Minister David Cameron chaired a meeting of the Government's Cobra emergencies committee to discuss the parcels, which sources said were "crude" in design but "could have caused injury to others".
A Number 10 spokeswoman said: "Seven suspect packages have been identified as containing small, crude, but potentially viable devices bearing the hallmarks of Northern Ireland-related terrorism. These have now been safely dealt with by the police and 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 yesterday, counter-terrorism officers said. One package was found in Aldershot, Hampshire, on Wednesday, while two were found on Tuesday at the armed forces careers office in Reading and the Army and RAF careers office in Chatham, Kent, the South East Counter Terrorism Unit said.
The official threat level is "severe" in Northern Ireland and in Great Britain "moderate", meaning an attack is possible, but not likely.