The Met, the largest force, topped the list, spending £5.2m from 2011-16, the data showed.
One critic said it did not stop the "cycle of crime", however, the National Police Chiefs' Council said it was "a cost effective tactic".
A Home Office spokesman said it was "an operational matter for police".
Informants can get anything from a few pounds for basic information, up to several thousand pounds for helping break up organised crime.
Payments are typically used to solve - or prevent - crimes including murder, terrorism and serious sexual offences, and can also be made to recover valuable stolen items.
Ronnie Howard, a former undercover police officer, said he made regular payments to informants, and in one case paid out £15,000 over 12 months in order to recover £3m worth of cannabis.
He said informants were usually recruited after being arrested and offered either money or a possible sentencing deal in court in return for information supplied.
Figures obtained by 5 live Breakfast, under a freedom of information request revealed 41 forces in England and Wales collectively paid £19.59m to informants between 2011 and 2016.
The Police Service of Northern Ireland spent almost £2m during the same period while Police Scotland, who only released figures for 2013-16, revealed it had spent £565,248.
There are no statistics available to show how many convictions have come as a result of paying informants - or covert human intelligence sources as they are officially known - but the National Police Chiefs' Council defended the practice.
Deputy Chief Constable Roger Bannister said: "The intelligence provided helps to prevent and solve the most serious of crimes and is vital in bringing offenders to justice through the courts.
"This is a well-established and highly regulated, worthwhile, and cost effective tactic, with the money paid to informants being very closely scrutinised."
But, Neil Wood, who worked as an undercover police officer and ran many informants, said the tactic has its limitations.
"It can be effective for certain crimes but for others - such as the war on drugs - using informants merely ensures that the cycle of violence and brutality continues," he said.
"Nobody wants to inform on the drug lords because of fears of violent reprisals, so it's only the low-lying fruit that gets caught out - and the trade continues regardless.
"Nobody can call that effective. Overall it does little to bring down the level of overall crime."
Of those which responded, 39 forces in England and Wales provided annual breakdowns, which suggested payments had fallen from £4.1m in 2011/12 to just over £3.1m last year.
North Wales Police paid out the smallest amount, handing over just under £40,000 over five years.
South Yorkshire Police and Cleveland Police did not provide figures.
Police forces are audited on their use of informants and are inspected annually by the Office of Surveillance Commissioners to ensure they are not breaking the law.
A Home Office spokesman said decisions on the operational deployment of resources "are matters for chief constables", adding: "There is no question that the police still have the resources to do their important work.
"We have provided a fair provisional funding deal for 2017/18.
"It provides stable funding for the police and enables essential policing reform and transformation to go further and faster so that we help the vulnerable, cut crime and support our communities."
Police forces across the UK paid out at least £22m to informants over the last five years, according to figures obtained by BBC Radio 5 live.