The Liberal Democrats and several campaign groups have expressed concerns about the amount of public money being spent without accountability on police informers.
By Chris Summers
Several people who work or have worked with the police have contacted the BBC News website to voice their concerns about the issue.
One man, who works with the police and has intimate knowledge of the system, said it was "open to abuse".
Jim, whose name we have changed, told the BBC News website expenses were frequently fiddled.
"I know of officers who have run non-existent sources and claimed the money themselves. Some use it for Friday afternoon drinking sessions.
"Within my unit there are sources who are paid on a week-to-week basis and get bigger payments when they get a major result, for example when they lead officers to hidden firearms.
'Turning a blind eye'
"A lot of units turn a blind eye to these sources committing crimes. In fact they sometimes say to them, 'Just make sure you don't get caught'.
"The Association of Chief Police Officers produced a document about the handling of sources, but it has never been published. The system badly needs regulating."
Steven - again it is not his real name - a former detective who used to handle covert human intelligence sources (CHIS) but now lives abroad, said it was inherently difficult to deal with people who were often liars and criminals.
He told the BBC News website: "It is a murky and complex and unsavoury world. Most of these people lie for a living - the trick is to find the lie... and the truth."
One horrific example of such duplicity was outlined in Liverpool Crown Court in 1999.
Kevin Morrison was eventually jailed for life for the robbing, mutilating and murdering 74-year-old Alice Rye.
Morrison, a registered police informant, had earlier tried to "whet the appetite" of Merseyside detectives by divulging previously unpublicised details about her death.
Morrison, who was seeking the £5,000 reward which was on offer, tried to incriminate an acquaintance, Keith Darlington.
Kevin Morrison tried to sell police information about his own crime
Steven told the BBC News website: "While historically some sources may have been on a retainer, in my experience this kind of thing was very rare indeed.
"They are usually paid on results - the better the information, the better the result. Poor, weak and misleading information, leads to a confused or poor result, and a lower payment.
"Having sources on the inside of a job is also a very risky business, for the police at court but also for the source themselves.
"Having a CHIS exposed in court means a massive expense for the police in terms of constant witness protection, relocation, new identities and so on.
"There are very high levels of control for this sort of thing - at detective chief superintendent level - given the risk assessment requirements for both the handler and the CHIS.
"The days of back-handers to worthless sources paid out by grubby detectives are long gone thankfully."
But Dick Kirby, a retired Flying Squad officer who ran informants throughout his career, said politicians were wrong to try to delve too deeply into how much money was being spent on informants.
He said: "The use of informants - and I include supergrasses - is one of the most productive ways of infiltrating, breaking up and prosecuting to conviction gangs of criminals. During my career, not one of my informants was ever compromised, nor even suspected.
"Matters are somewhat different, now. There is far too much paperwork generated regarding the true identity of informants, thus raising the risk of identifying them. And now, this latest move by politicians to get the various police forces to reveal how much is spent rewarding informants - I can tell them, not enough!"
Mr Kirby, who has written several books on the subject, said: "Grassing has always been a high-risk business. Why anybody would wish to do it nowadays is beyond me, yet they do.
"I cannot see any sense in revealing how much is spent on informants. It is simply a case of politicians wanting to know, for the sake of knowing. It would also be another nail in the coffin for informants, to lead to their identification."