The Metropolitan Police has been given £22.5m over the past five years by different businesses and organisations.
A Freedom of Information request by the BBC has revealed that payments made to the force include remuneration from G4S for an operations room, local businesses giving mountain bikes, and oil companies paying for extra policing of petrol stations.
The Met says the extra funding allows them to do more, but does not allow companies to interfere with their duties.
Dr Tim Brain, honorary senior research fellow at Cardiff University and a former chief constable of Gloucestershire Police, told Today presenter Sarah Montague that "there is no problem with this" and it is "allowed by law".
"We have to see this in context," he added, "this is £22.5m over 5 years; the Met Police budget is £3.5bn annually."
Jenny Jones, Green Party Assembly member and deputy chair of the Police and Crime Committee at the London Assembly, asked for "more information" from the met: "Some of this looks like rent-a-cop policing which the majority of the public would not find acceptable."
While echoing Dr Brain, she cautioned that "this is such a small amount of money - they shouldn't even go there."
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