Counter-terrorism police are investigating reports of suspicious packages being found at Army offices around the country.
Terrorism police are investigating a total of six suspected bombs sent to Armed Forces recruiting offices.
Prime Minister David Cameron has chaired a meeting of the Government's Cobra emergencies committee to discuss the suspect packages, Downing Street said.
Packages have been found at Oxford, Slough and Brighton, the South East Counter Terrorism Unit said.
The finds come after similar packages were discovered on Wednesday in Aldershot, Reading and Chatham.
Ministry of Defence bomb disposal units have been called, although this is routine with any suspect package.
A Ministry of Defence spokesman said: "We can confirm that we are aware of security incidents ongoing in Oxford, Slough and Brighton involving armed forces careers offices.
"The relevant police authorities are investigating with military EOD (explosive ordnance disposal) teams in support and any further inquiries should be directed to the police.
"Security advice has been reiterated to our personnel."
Police officers consider the situation a "low-level threat" but "viable", sources said.
Queensmere Shopping Centre, in Slough, was evacuated, a spokesman for the centre confirmed, while St Giles in Oxford was closed.
It is understood a suspicious package was also reported at the army careers office in St Peters Street, in Canterbury, Kent.
Officers have set up a police cordon and shoppers are being told to avoid the area, the Kentish Gazette reported.
Elsewhere, it emerged that 100th Security Forces Squadron personnel activated special control measures at RAF Mildenhall in Suffolk at 8.20am this morning after finding a suspicious package in a vehicle entering the site during a routine inspection.
The incident has not been formally connected to the packages found at the careers offices.
Lieutenant Colonel Kevin Parker, 100th Civil Engineer Squadron commander, wrote on the website for the base that the squadron, along with the Ministry Guard Service, initiated enhanced security measures after discovering the package in a vehicle at the main gate.
The Explosive Ordnance Disposal unit was called to the scene to investigate the package. The gate reopened at 11am after the area was secure.