Suspect packages sent to armed forces recruitment offices bear "hallmarks of Northern Ireland-related terrorism", a Number 10 spokeswoman has 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.
The national threat level remains under constant review.
More top news
Mild but cloudy
On his 93rd birthday, Robert Mugabe insisted he has no plans to stand down as Zimbabwe's leader and vowed to stand again in the election.
A Muslim teacher, who was denied entry to the US, has told ITV News he was 'distraught' when he was removed from the flight.