The Police Service of Northern Ireland has paid £1,995,392 to informants over the last five years, data shows.
Figures obtained through a BBC Radio 5 Live freedom of information request revealed that more than £19.59m was spent collectively by 41 forces between 2011 to 2016.
One critic said the tactic does "little to bring down the level of overall crime", but the National Police Chiefs’ Council (NPCC) said the intelligence is a "vital" tool to bring offenders to court.
Figures show the biggest spenders over the five years were Scotland Yard, who topped the list on £5.2m, with Kent Police paying £1,029,145 and West Midlands Police £1,002,834.
The Police Service of Northern Ireland spent £1,995,392, while North Wales Police spent just under £40,000 - the smallest amount.
Police Scotland only provided figures for 2013 to 2016 - with £565,248 paid.
The BBC said of the forces that responded, 39 in England and Wales gave annual breakdowns, with figures suggesting payments had fallen from £4.1 million in 2011 to 2012 to £3.1 million last year.
South Yorkshire Police and Cleveland Police did not provide figures, according to the BBC.
The Office of Surveillance Commissioners inspects forces to ensure they are complying with legislation when using informants - also known as covert human intelligence sources.
Former undercover police officer Neil Wood said: "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.
"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. It does little to bring down the level of overall crime."
The NPCC lead for covert human intelligence sources, Deputy Chief Constable Roger Bannister, said: "The use of informants to assist in investigations is one resource used by police forces across the country to defend and protect the public.
"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 tactic with the money paid to informants being very closely scrutinised."
The Home Office said decisions on the operational deployment of resources are matters for chief constables, in association with police and crime commissioners.