The British government held an emergency meeting on Thursday after a string of crude but potentially viable explosive devices were mailed to armed forces recruitment offices, which it attributed to Northern Ireland insurgents.
The devices, sent to seven offices in southeast England, bore the hallmarks of Northern Irish terror attacks, Downing Street said.
Counterterrorism police are investigating and army bomb disposal crews were sent to assist. Sources said they could have caused injury.
Prime Minister David Cameron chaired a meeting of the government’s crisis committee, called Cobra, to discuss the situation.
“Seven suspect packages have been identified as containing small, crude, but potentially viable devices bearing the hallmarks of Northern Ireland-related terrorism,” a Downing Street spokeswoman said afterwards. “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.”
An envelope was delivered to an office in Chatham and a package was received in Reading on Tuesday. Another was found on Wednesday in the army town of Aldershot.
Four more were delivered on Thursday in Brighton, Canterbury, Oxford and Slough, the police’s South East Counter Terrorism Unit (Sectu) said.
One of the packages was posted from the Republic of Ireland, sources said.
“The devices will undergo forensic examination,” said Sectu’s Detective Superintendent Stan Gilmour. “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.”
Police said all military facilities and post offices had been told to be “extra vigilant.”
A shopping centre in Slough to the west of London was temporarily evacuated, while cordons were placed close to all offices where packages were found. “It is a necessary precaution until we know what we are dealing with,” Gilmour said.
The Royal Mail postal operator said it was co-operating with the police investigation.
The Downing Street spokeswoman said the national threat level remained “under constant review”.
Irish Republican Army dissidents opposed to the peace process have mounted gun and bomb attacks in Northern Ireland in recent years, killing two soldiers, two policemen and one prison officer since 2009. In October they were blamed for sending letter bombs to high-profile political and security figures in Northern Ireland.
But they have not attacked mainland Britain since 2001, when car bombs outside the BBC headquarters, a London nightspot and other buildings wounded several people.
According to the website of MI5, Britain’s spy agency, the current threat level for Northern Ireland-related terrorism is severe in Northern Ireland (an attack is highly likely) and moderate in Great Britain (England, Scotland and Wales – wherein an attack is possible, but not likely).
Dissident republican paramilitary groups reject the peace process in Northern Ireland and continue to carry out attacks, with the intention of destabilising the province and the power-sharing arrangements between the Protestant British and Catholic Irish communities.
Meanwhile, in an incident thought to be unrelated, a suspicious package was found during a routine vehicle search at the gates of Mildenhall air station in eastern England, one of the major US military airbases in Europe. The item was determined to be a home-made firework.
A spokeswoman for the base said the incident “is not thought to be terrorist-related”.