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.
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Another mild day with highs reaching up to 13C degrees in mild southerly winds