UK police forces paid more than £6m in the past financial year to people with information on criminal activity, BBC Radio 5 Live has discovered.
The Met Police spent most at £1.86m, followed by Greater Manchester Police at £329,497 and the Police Service of Northern Ireland at £299,000.
The Association of Chief Police Officers said the system was "vital in bringing offenders to justice".
The figures emerged for the first time after a Freedom of Information request.
Individual police forces have refused to go into specific detail of how the money was spent, but say it was vital in tackling a whole range of criminal activity.
The forces with the largest bills for "covert human intelligence sources" - as informants are officially termed - include West Midlands Police, which spent £291,780 in 2008/9; Strathclyde, which paid out £221,598.24; and Northumbria, whose bill was 191,652.56.
The median figure for payouts by Kent Constabulary since 2001/2 was £222,578.
One former superintendent who worked for more than 20 years as an informant handler told the BBC that most informers earned between £50 and £2,000 for information - though a select few had been paid more than £100,000 a year, for vital intelligence.
BBC Radio 5 Live's Gavin Lee said that although the total spent by the police is more than £6m, the true scale of the informer system across the security services is greater than this figure suggests.
Criminal informants can also be offered police help to reduce a potential prison sentence.
Patricia Gallan, Assistant Chief Constable of Merseyside Police and chairman of ACPO's National Source Working Group, said the use of informants had proved essential in cases ranging from serious organised crime to burglary.
"Each force is audited on their use of informants and is subject to a robust annual inspection by the Office of Surveillance Commissioners to ensure compliance with the law," she added.
"They are a valuable source of intelligence and their use is justifiable and proportionate when set against other police tactics."
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