Former IRA men have helped dissidents republicans in their attempt to destabilise the peace process, according to the Independent Monitoring Commission.
In its latest report published on Wednesday, the monitoring group said the threat was the highest it had seen over the last six years, as dissidents work more closely together.
"There are indications that former republican terrorists have, as individuals, provided services in some instances to dissident republican groups, which, even if occasional, can significantly add to the threat," the IMC said.
"Some activities have involved co-operation between individuals from different groups without there being strategic co-operation by their leadership," it added.
The IMC said dissidents remained committed to launching an attack in Britain if the opportunity emerged.
The monitoring group, which reports to the British and Irish Governments on the activities of paramilitary groups, said they posed a major challenge to security forces.
It added: "The overall level of dissident activity was markedly higher than we have seen since we first met in late 2003.
"The seriousness, range and tempo of their activities all changed for the worse in these six months."
The latest IMC report covers the period from March to August this year.
"During this period dissident republicans were violent and showed an intent to kill if the opportunity arose."
In March the Real IRA shot dead two soldiers, Patrick Azimkar, 21, and Mark Quinsey, 23, outside the Massereene Army base in Antrim.
Within two days the Continuity IRA murdered Constable Stephen Carroll, 48, in Craigavon.
The IMC said: "The three murders in March were by far the most serious incidents but there were many others involving extreme ruthlessness."
The organisation cited a string of bomb and gun attacks launched by the groups, noting that many could have easily claimed lives.
"A number of the incidents for which dissidents were responsible demonstrated a capability to plan and organise which adds to the threat they present," said the IMC.
"They pose a major challenge to the law enforcement and other agencies on both sides of the border."
Devolution
The IMC said dissidents could be undermined by the devolution of policing and justice powers to Stormont, arguing that the move would signal the strength of the political process.
The Republic of Ireland's Minister for Justice Dermot Ahern said the IMC comments on the political process were a useful contribution.
He also hit out at dissident groups: "These are a small hardcore of unrepentant criminals who have no support amongst the Irish people, North or South."
Alliance leader David Ford said there were no surprises in the latest IMC report.
"What I do think we have is a clear wake up call to politicians in the assembly that they can't drag their feet any longer.
"They cant play silly games fighting over parades and so they must really get down to the issue of devolving justice," he said.
"The report makes it clear this is one way of dealing with the dissident threat," he added.
Secretary Of State Shaun Woodward told the Commons:
"This report is very, very clear about the problems we are facing in Northern Ireland today as a result of criminals calling themselves dissidents.
"It's very, very clear that there is a political solution and what they advocate is that actually early devolution of policing and justice powers to the Northern Ireland assembly could provide a potent intervention."
DUP deputy leader Nigel Dodds said: "The terrorist threat which we all face in Northern Ireland highlights exactly why it is so important to ensure the conditions are right before any transfer of policing and justice powers.
"Whilst other parties were urging the DUP to agree an incomplete financial package, we were pressing the Government to ensure that the finances would be there to help tackle such problems.
"We will continue to press ahead, however, in ensuring there is community confidence prior to any transfer of these sensitive powers."
The IMC also assessed the decommissioning of loyalist weapons.
Decommissioning moves by the UVF and UDA were welcomed, but the IMC said members of both organisations were still involved in crime.
It said UVF members were also involved in events which led to the murder of Catholic community worker Kevin McDaid in Coleraine.
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