A majority of convicted terrorists imprisoned since the 9/11 attacks have reportedly been released from prison and still hold their extremist beliefs.
Of the 583 people imprisoned on terror charges in the last 15 years, 418 have been released, a Sky News investigatoin found.
Two-thirds of those released refused to take part in prison deradicalisation programmes aimed at addressing their extremist convictions.
It comes as the head of Mi5 said police and intelligence services have prevented 12 terror plots since June 2013.
Andrew Parker, who has been director general of the UK security service for nine years said there was a "large homegrown problem of potentially violent extremists in the UK," estimating they were "about 3,000 in number".
The spy chief said the number of potential extremists living in the UK was one of three aspects contributing to the wider Islamic extremist threat - along with the members of Isis operating in Syria and Iraq and the group's attempt to spread its "toxic ideology" online.
His comments came after the National Crime Agency (NCA) and Scotland Yard's counter-terrorism command launched a joint campaign urging the public to report any fears of criminals handling firearms.
The Sky investigation revealed that 104 convicted terrorists were released in the last year after serving sentences between 12 months and four years - a typical sentence for someone jailed for supporting and encouraging terror groups or attacks.
Another 24 were released after serving more than four years, meaning they are likely to have played a more important role in terror planning.
Also among those released were three men who assisted the London suicide bombers in planning the 7/7 attacks in 2005, along with five people who plotted a dirty bomb attack in London in 2004.
Lord David Blunkett, told Sky News there needed to be a more robust programme to monitor released terrorists.
"It's perfectly reasonable to say that once someone's served their sentence, if it isn't possible to reassess them, we should continue to monitor them outside prison," he said.
"So, if there's any indication at all that they are reconnecting with organised terrorist groups, the intervention can take place very quickly rather than allowing them to commit another act and then having to try to pick them up again."
