Fears of a fresh terror threat are growing after Downing Street revealed suspicious packages sent to armed forces recruitment offices bore "hallmarks of Northern Ireland-related terrorism".
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, but sources told the Press Association one of the packages was stamped with a Republic of Ireland (RoI) postmark. It is believed dissident republicans opposed to the peace process are behind the intimidation attempt.
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."
Defence Secretary Philip Hammond said the Government had not "taken its eye off the ball" with Northern Ireland, adding that it remained "very high on our list of security priorities".
A Number 10 spokeswoman said yesterday: "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."
Detective Superintendent Stan Gilmour of the South East Counter Terrorism Unit (Sectu), said the contents of the packages were "suspicious in nature" and would be forensically examined.
He said: ''Even if the contents are determined to be a viable device, they pose a very low-level threat and are unlikely to cause significant harm or damage."
Advice has been sent to the Royal Mail and to the Ministry of Defence to ensure that staff ''remain vigilant'', Sectu added.
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.
Mr Hammond told Iain Dale on LBC: "Those of us living in Great Britain on the mainland, it's a long while since we've seen Northern Ireland-related terrorist activity on the mainland.
"But of course for those living in Northern Ireland, this problem hasn't gone away and there has been a continuing drum beat, albeit of course at a much lower level than it was in the past, but a continuing drum beat of activity from dissident Republican organisations.
"I can assure you we haven't taken our eye off the ball. Northern Ireland remains very high on our list of security priorities and the various security authorities are very conscious of the fact that there is a continuing terrorist threat from dissident Republican organisations and that it could spill over into the UK mainland, and we keep a very close watch on that."
Shadow Northern Ireland secretary Ivan Lewis said: "These devices bear the hallmarks of another attempt by dissidents to reverse the progress we have seen in Northern Ireland over the past 15 years.
"Their attempt to harm innocent people will be condemned by the people of Northern Ireland, including by those they claim to represent.
"We will give our support to the Government in taking the necessary steps to bring those responsible to justice."
Democratic Unionist deputy leader Nigel Dodds condemned those behind the bombs, saying: "Northern Ireland has turned a corner. We are moving forward and no-one wants to go back to the bad old days."
Ulster Unionist Party leader Mike Nesbitt said the deliver of the packages " must also be named for what it was, a series of acts of terrorism", while t he cross-community Alliance Party also condemned those responsible.
Alliance Assembly member Stewart Dickson said: " There is no justification for these attacks. Violence has no place in our society."
In October last year dissident republicans opposed to the peace process were blamed for sending a series of letter bombs - thought to be similar to the devices found yesterday - to high profile political and security figures in Northern Ireland.
One of the devices was sent to the seat of power sharing executive at Stormont Castle in Belfast addressed to Northern Ireland Secretary Theresa Villiers.
Another bomb was delivered to the offices of the Public Prosecution Service in Londonderry while two explosive packages - one addressed to Police Service of Northern Ireland (PSNI) chief constable Matt Baggott and the other to one of his senior commanders - were intercepted at Royal Mail offices in Belfast and Lisburn.
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.