LONDON -  Prisons in Europe are becoming "breeding grounds" for militant groups, with some criminals seeing violent extremism as a form of redemption for their crimes, a report by a British think tank published Tuesday said.

Militant and criminal groups are recruiting from the same pool of people, while their social networks are also converging, the International Centre for the Study of Radicalisation and Political Violence (ICSR) found, in what it dubbed a "new crime-terror nexus".

The emergence of the Islamic State group (IS) has strengthened the link between crime and terrorism, according to the report which examined the profiles of European militants recruited since 2011.

Rather than looking to universities or religious establishments, IS increasingly turns to "ghettos", prisons and "underclasses" to recruit individuals with a history of criminal behaviour, it said.

Prisons provide a ready supply of "angry young men" who are "ripe" for radicalisation, according to the study, entitled "Criminal Pasts, Terrorist Futures: European Militants and the New Crime-Terror Nexus".

ICSR director Peter Neumann, one of the report's authors, said the lines between crime and militant groups were becoming "increasingly blurred". "Prison is becoming important as a place where a lot of networking happens," he said.

"Given the recent surge in terrorism-related arrests and convictions... we are convinced that prisons will become more - rather than less - significant as breeding grounds for the militant movement."

Neumann said radicalisation was becoming faster because "a lot of these people have already been convicted of violent crime, so the jump to being a violent extremist is not so big." Recruiting in prisons allows militant groups to tap "transferrable skills", the study found, including familiarity with weapons and self-financing through crime.

Researchers from the ICSR, based at King's College London, compiled profiles of 79 European militants with criminal pasts, from Belgium, Britain, Denmark, France, Germany and the Netherlands.

All had either travelled abroad to fight or been involved in terrorist plots in Europe.  Over the past five years an estimated 5,000 Western Europeans have travelled to the Middle East to join militant groups such as IS and the Syrian Fateh al-Sham Front, a former Al-Qaeda affiliate, the report said.

Of those studied, 57 percent had been incarcerated before being radicalised and at least 27 percent of those who spent time in prison were radicalised behind bars.

For some, jihadism offered a form of "redemption" for their crimes, researchers said.

Ali Almanasfi, a British-Syrian from London who fought in Syria after serving a jail term for violent assault, was cited in the report as saying: "I want to do something good for once. I want to do something pure."

According to Neumann, the findings point to a shift if the way IS operates.

"We think IS no longer aspires to be a very theological organisation. It embodies the brutality, strength and power that these young people who were often members of gangs are looking for," he said.

"It basically tells them 'you can continue to do all the things you did before, but now you can get into heaven'."