SUSPECTED explosive devices sent to British Army recruitment offices were sent by the New IRA Alliance, it is believed.
Garda anti-terrorist officers are now working closely with the PSNI to combat the new campaign.
Seven crude explosive devices were sent to British Army recruitment centres over the past three days. And it has been established that at least one of them was mailed from a Border county in the Republic.
None of the suspicious parcels exploded and they were dealt with by British Army bomb disposal experts.
Last night, the packages were undergoing forensic tests and officers were awaiting further details on the postmarks.
Security chiefs on both sides of the Border believe the parcels were sent by associates of a senior New IRA Alliance figure, based in Derry city.
This man is the former leader of the Real IRA in the city and had links in the past with the group led by murdered Dublin terrorist Alan Ryan.
The man had previously boasted that his gang would launch a campaign of violence in Britain, but so far their activities have been confined to the North.
The PSNI believe these latest attacks are linked to a letter bomb campaign in Belfast and Derry last October.
Four parcels were discovered at British Army careers offices in Oxford, Brighton, Canterbury and the Queensmere shopping centre in Slough yesterday.
Another package was found in Aldershot, Hampshire, on Wednesday while two packets were intercepted on Tuesday at careers offices in Reading, Berkshire, and the British army and RAF recruitment building in Chatham, Kent. Following the discovery of the packages, Mr Cameron chaired a meeting of the British government's Cobra emergencies committee to discuss the threat posed by the new campaign.
The packages were said to be crudely designed but potentially viable and could have caused injury if they had exploded when opened.
The New IRA Alliance was formed in the summer of 2012 when four established dissident groups came together.
These included several factions of the Real IRA, the ONH faction, Derry-based Republican Action Against Drugs and another group comprised of former members of the Provisional IRA.
Gardai are also helping with the attempts to identify the postmarks on the packages and checks are being made with mail centres in a number of Border counties.