The Defence Secretary has said the Government had not "taken its eye off the ball" with Northern Ireland, amidst fears that a fresh terror threat is growing.
Philip Hammond has insisted that Northern Ireland remained "very high on our list of security priorities" after Downing Street revealed suspicious packages sent to armed forces recruitment offices Thursday bore "hallmarks of Northern Ireland-related terrorism".
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Four suspected explosive devices were discovered at army careers offices in Oxford, Brighton, Canterbury and the Queensmere shopping centre in Slough yesterday, anti-terror police said.
The shopping centre was temporarily evacuated, while cordons were placed close to all offices where packages have been found. Ministry of Defence bomb disposal units were also called.
No specific group has claimed responsibility.
Prime Minister David Cameron 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".
The latest deliveries follow packets sent to Aldershot, Hampshire, on Wednesday and another two on Tuesday to an armed forces careers office in Reading and the Army and RAF careers office in Chatham, Kent.
Details of the letter bombs emerged hours after a pipe bomb exploded in a residential area of Newry. No one was injured in Wednesday night's blast in the Co Down town.
The latest threats have drawn widespread condemnation.
Stormont Deputy First Minister Martin McGuinness condemned those who continued to engage in violence.
Sinn Fein's Mr McGuinness said pipe bombs and letter bombs were an "attack on the peace process". He said on Twitter: "Those responsible belong to the past. Their futile acts must be condemned."
A Number 10 spokeswoman said "the national threat level remains under constant review."
Sources in Dublin indicated that two of the seven low-grade explosive parcels were sent from Ireland.
The others, it is understood, were delivered from different locations within Britain. Although they were not sophisticated, they were described as viable.
The official threat level for Northern Ireland-related terrorism is set separately for Northern Ireland and Great Britain, that is, England, Wales and Scotland.
In Northern Ireland it is "severe" and in Great Britain "moderate", meaning an attack is possible, but not likely.
The spate of letter bombs marked the re-emergence of a terror tactic that was used by paramilitaries during the Troubles.
Since 2009, violent republican extremists have murdered two soldiers, two policemen and one prison officer in Northern Ireland.