New figures show the Metropolitan Police earned almost £23 million in sponsorships from dozens of organisations, including oil companies, over the past five years.
Scotland Yard officers also benefited from gifts including football shirts, pop concert tickets and luxury cars.
Commentators said the figures provided an insight into some less well-known sources of income as the force grapples with sweeping budget cuts.
While such sponsorships were within the rules, critics today questioned whether they "could be perceived as compromising the force's position".
The Met Police defended the disclosures, made under Freedom of Information laws, insisting that every gift was "subject to rigorous parameters".
According to the new figures, the Met received more than 830 payments, worth at least £22.7 million, between April 2007 and March this year.
Dozens of police stations and units across London were beneficiaries, with some donations used in local crime reduction projects, the BBC reported.
The biggest donor was the Association of Payment Clearing Services, which represents credit card firms.
It made almost two dozen payments totalling almost £12 million on a fraud investigation unit called the Dedicated Cheque and Plastic Crime Unit.
Detectives from the unit work with bank fraud investigators and are fully sponsored by the banking industry, which invests nearly £5m annually in the operation.
The British Oil Security Syndicate, a private security organisation which represents forecourt owners and fuel retailers, paid more than £30,000 for a dedicated officer to fight crime at petrol stations.
Other donations included football shirts provided by Queens Park Rangers and Chelsea to support vehicles for the royal protection and special branch donated by BMW and Land Rover UK.
Local businesses across the force's 32 boroughs also donated patrol mountain bikes to help police their areas. Meanwhile record company EMI donated 11 concert tickets to officers. No further details have been made public.
Association of Chief Police Officers guidance says that private income can "help forces counter the effects of declining budgets and increasing pressure on resources".
Sponsorship is subject to a one per cent limit of force's total annual income and the acceptance of sponsorship for "non-core police activities" helped the force's service to the community. But critics today questioned the deals.
Keith Vaz, the chairman of the Home Affairs Select Committee chairman, told the BBC that the Met should "consider very carefully" whether any gifts "could be perceived as compromising the force's position".
He said his committee was currently holding an inquiry into "leadership and standards in the police" and that it would "be hearing from the commissioner on these matters".
This week, the Met announced surprise plans to sell its current central London headquarters at New Scotland Yard, home for almost half a century, in order to save £6.5 million a year.
The radical fire-sale plans are part of drastic budget cuts that will also involve axing dozens of police stations and buildings across the capital.
The force is attempting to save more than £500 million from its annual budget of £3.6bn by 2015, in the wake of government cuts.
Jenny Jones, a member of the Metropolitan Police Authority, told the BBC: "Some of these [arrangements] look like rent-a-cop policing that the public might not find acceptable."
Today, a Met Police spokesman defended the funding, insisting that money from outside organisations was used to positive affect in several areas of the force.
He added that Scotland Yard had a "long history" of working with different partners to tackle crime.
"Donations and sponsorship over £50,000 must be referred to the Deputy Mayor for Policing and Crime for approval," he said.
"Such arrangements are subject to rigorous parameters.
"They do not make any of the statutory functions of the MPS dependent on this funding nor does it allow for any companies to interfere with the duties of the police."