Police forces across Britain have been urged to reveal how much public money has been paid to informers.
Seven police forces rejected requests from the BBC, made under the Freedom of Information Act, to reveal how much taxpayers' money was paid out.
The Lib Dems and campaign groups have expressed concerns about public money being spent without accountability.
In September a man jailed for life for two murders was revealed to have been on Greater Manchester Police's payroll.
'Public should know'
Liberal Democrat home affairs spokesman Nick Clegg said: "The refusal of police forces to provide basic information on the use of taxpayers' money is difficult to understand.
"It is not as if we have been asking for operational details on the informants used by the police which, understandably, should remain as confidential information.
"But surely the public is entitled to a rough idea on the amount of public money spent on informants, in the interests of transparency"
Police forces, MI5 and HM Revenue and Customs are all believed to have increased their use of covert human intelligence sources (CHIS) in recent years as they seek to combat terrorism and organised crime.
In 2005 the government introduced the Serious Organised Crime and Police Act, which encourages using such people to bring down criminal gangs.
But some lawyers and civil liberty campaigners fear there is insufficient scrutiny of payments to informers and a lack of public accountability.
Rodney Warren, of the Criminal Law Solicitors Association, said the use of registered police informants was widespread and he said he knew about a case in which the Court of Appeal had ordered a retrial, at huge public cost, because the prosecution had failed to disclose that a key player in the case had been on the police payroll.
He said: "That highlights the dangers there are when an informant is paid for information."
BBC News Interactive approached the Metropolitan Police, the Police Service of Northern Ireland and forces in Greater Manchester, Strathclyde, West Midlands, South Wales and Essex but all refused to supply any information, claiming there was an exemption under the Freedom of Information Act.
Former Home Secretary John Reid wrote to the Liberal Democrats and said: "There is scrutiny of this process...There will be records within each agency making payments to sources but there is no centrally aggregated figure.
"It is open to chief officers to make details of payments made to sources available to the police authority and for the police authority to scrutinise the payments made to sources as part of its audit and assurance responsibilities."
Jenny Jones, a Green councillor and member of the Metropolitan Police Authority: "I did not realise that the Metropolitan Police Authority could scrutinise these accounts and I will be requesting that they do. It is unbelievable that these payments do not get scrutinised in some way by a publicly accountable body."
There have been concerns about the use of informers ever since 1973 when Britain's first supergrass, armed robber Bertie Smalls, was given complete immunity in return for giving evidence which convicted his accomplices.
But the criminal landscape has changed a great deal since then and terrorism is now a much bigger problem than armed robbery.
John Toker, the government's chief spokesman on counter-terrorism, told the BBC: "There is a global figure for the amount spent on the security services, which is going up, but we would not break it down into what is spent on covert human intelligence sources.
"It is part of the government's over-arching policy not to discuss the details of such matters."
'Why not be open about it?'
Corin Taylor, from the Taxpayers' Alliance, conceded that the police often had to deal with "shady characters" but he said: "The police ought to be open about it and if they think that it's a justified use of taxpayers' money they should tell us how much they have spent. If they have got nothing to hide they should be open about it."
In September Glasgow-born gangster Stephen McColl, known as Boom Boom, was convicted of two murders. The Guardian newspaper said he had been providing information to Greater Manchester Police's robbery squad.
His crimes were so serious he was told by the judge, Mr Justice Henriques, he would never be released from jail.
Value for money?
Another case which has caused concern was that of a former UVF commander in Belfast, Mark Haddock, who was paid at least ?80,000 by his police handlers between 1991 and 2003.
In January the Northern Ireland police ombudsman, Nuala O'Loan, said there was evidence Haddock, who is in jail for a violent assault on a nightclub doorman, and several other UVF informants, were linked to 10 murders, a bomb attack, drug dealing, extortion and intimidation.
In recent years there have been a number of other cases where criminals have come forward and done deals with police in exchange for financial inducements.
Maurice Frankel, from the Campaign for Freedom of Information, said the forces' responses had been "not very impressive" and he said they should explain why they could not provide the figures rather than just throwing a "blanket of secrecy" over the whole subject.
He said: "It's quite likely that the spending is going up as the police become more focused on organised crime and terrorism but you just don't know.
"Even if there is a trend the question is: 'Are they getting value for money for that spending?'"
'Power to induce'
Jack Whomes is serving a life sentence for three murders after being convicted on the word of supergrass Darren Nicholls.
It later emerged that Nicholls had organised a lucrative book and TV deal while he was in custody preparing to give evidence.
From his cell at Long Lartin prison in Worcestershire, Whomes said: "Even someone like me, who has been on the receiving end, fully believes that the public has a duty to help the police with providing crime intelligence.
"But can it be right for police officers to have the power to induce, with public funds, one suspect to give evidence against another suspect who the police officer thinks is the most guilty?"