Four suspect packages delivered to army careers offices bear "the hallmarks of Northern Ireland-related terrorism", Downing Street has said.
Packages were sent to armed forces recruitment centres in Oxford, Slough, Kent and Brighton.
The latest deliveries follow letter bombs sent to offices in Hampshire, Kent and Berkshire earlier this week.
The threat was discussed at a meeting of the Cobra emergencies committee, chaired by the prime minister.
A spokesman for Number 10 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 Ministry of Defence (MoD) said it was aware of "security incidents" and army staff were warned to be "extra vigilant".
Packages were found at offices in St Giles, Oxford; the Queensmere Shopping Centre in Slough; St Peter's Street in Canterbury and Queens Road, Brighton.
Det Supt Stan Gilmore, of the South East counter-terrorism unit, said the packages found on Thursday would be sent for examination.
"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," he said.
Democratic Unionist Party MP Nigel Dodds condemned those behind the packages.
"Those who cling to terrorism should realise that it failed in the past and it will do so again. It will only lead to further hurt and suffering," he said.
"Northern Ireland has turned a corner. We are moving forward and no-one wants to go back to the bad old days."
Ivan Lewis, Shadow Secretary of State for Northern Ireland, 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."
Stormont Deputy First Minister Martin McGuinness condemned those who continued to engage in violence.
Commenting on Twitter about the letter bombs and a pipe bomb which exploded in Newry without causing injury, he said they were "an attack on the peace process".
"Those responsible belong to the past. Their futile acts must be condemned," he said.
On Wednesday officers said they believed letter bombs sent earlier in the week to careers offices in Chatham and Reading were linked.
A device was also found at the office on Hospital Hill, Aldershot.
The packages received on Tuesday were forensically examined after being made safe.
Police in Reading described the bomb found at St Mary's Butts as a "small but viable explosive device".
An MoD spokesman said: "Security advice has been reiterated to our personnel."