The use of "supergrasses" in the fight against crime could be threatened by a new act of Parliament, a gathering of academics and police officers at Durham University heard this week.
Alisdair Gillespie, director of the Centre for Police Research and Education at Teesside University, said that the use of general informers and undercover agents had been placed on a statutory basis for the first time and that the use of criminal informers was now a grey area.
The problem lay in the Regulation of Investigatory Powers Act that came in to force earlier this year to help meet the challenges of the Human Rights Act. But the RIPA may not have gone far enough in regulating the use of criminal informers, according to Mr Gillespie, probably because Parliament did not want to be seen to be "licensing crime".
At the conference, entitled Covert Policing and Human Rights, he warned that a challenge in the courts was likely.
In the new human rights era, the activities of informers could be problematic because they gather private or personal information.
"This practice is now open to legal challenge and may become illegal," Mr Gillespie warned. If so, law enforcement agencies would lose one of their most powerful weapons against crime.
The Home Office needed to clarify the position urgently, he said.