There is a "hugely inconsistent approach" across police forces in their attitude towards free gifts, a report found today.
While major contracts were professionally managed, "checks and balances are less evident on spends of around £5,000 and under", a report by Her Majesty's Inspectorate of Constabulary (HMIC) found.
But the inspectors added that corruption was not endemic in the police service.
Inspector Roger Baker, who led the review, said: "While we found no evidence of endemic corruption in police service relationships, we did find significant variations between forces and authorities in how they defined what is acceptable and what is not.
"This inconsistency made little sense to us and nor do we believe would it to the general public.
"There are no geographic boundaries when it comes to integrity and there should not be local differences in standards."
An assessment by the Serious and Organised Crime Agency (Soca) found "the most significant threat nationally" was "information disclosure to those involved in organised criminality, to friends and family, and to private investigators".
But today's review said: "It was disappointing to note that whilst this had been flagged as a threat, HMIC found one force had authorised secondary employment for three officers working as private investigators.
"At least one of these had authority to investigate accidents for private insurance companies."
The report also found that forces were "outsourcing much of their back-office activity as well as procuring goods and services annually worth hundreds of millions of pounds".
"HMIC has seen instances where police officers and staff who have led on negotiations with suppliers have, upon successful conclusion of contracts, left the service and been immediately employed by that contractor - in one case as contract manager for the company's relationship with the force."
It found police forces had no clauses which would prevent contractors from employing anyone, directly or indirectly, who had served with the force for the duration of the contract.
But it said "there would certainly be merit in doing so".
"This would avoid the perception of conflicting priorities as a result of post-service 'revolving doors'," the report said.
Only 20 of the 43 forces in England and Wales gave staff clear written guidance to help them decide whether to accept or decline a gift, with 15 placing an acceptable value on gratuities of between £5 and £75.
"All forces and authorities have a recording mechanism for gratuities and hospitality: but these are not consistently completed in most cases," the report said.
"There are many examples of departments and BCUs (basic command units) not recording anything at all (even though focus groups and interviews confirmed that hospitality had been received).
"There is also evidence of officers recording the receipt of gifts in their pocket books rather than in the formal registers."
Mr Baker added there was "an urgent need for a wake-up call for the police service and its leaders".
The report found there was a need for police to recognise that it was not just important to act fairly, but also to be seen to be acting fairly.
The lack of controls, not always considered as corruption, can allow a slippery slope to develop, it added.
But concerns around inappropriate relationships with the media were seen as a problem for only the Metropolitan Police in London.
Many forces appeared complacent, adopting the attitude that "it would not happen here", the report said.
There was also "little clarity about the boundaries of acceptability" in relation to corporate entertaining with the media, with many forces relying on a common-sense approach.
The report said: "We found that forces lack the capacity and capability to proactively identify any inappropriate relationships.
"Forces conveyed a sense of inevitability that resourcing complex investigations into media leaks rarely yields any positive results.
"Forces should explore options for identifying and monitoring emerging and inappropriate relationships with, and leaks to, the media."
The inspectors also called for forces and authorities to record "all interactions between police employees and media representatives".
The 14-week review was ordered by Home Secretary Theresa May in July.
Chief Constable Mike Cunningham, the lead on professional standards for the Association of Chief Police Officers (Acpo), said: "The police service is a highly accountable organisation which must be responsive and trusted by the public.
"With a dedicated team to investigate allegations of corruption in each force, we have one of the least corrupt police services in the world.
"This review bears out research from other bodies such as Transparency International, who found no evidence of systemic corruption in policing, but individual cases where police officers let their colleagues and the public down."
He went on: "Leadership has always been an important part of tackling police corruption and the service has worked hard to identify threats and put preventive measures in place.
"All our relationships must meet the highest standards of integrity and this review highlights the need to continue to develop safeguards and to keep pace with new developments in information technology which expand the potential for vulnerability to corrupters.
"Any officer, regardless of rank, that brings the service into disrepute does huge damage to the 140,000 officers that go out every day to deliver a police service with commitment and integrity."
Mark Burns-Williamson, chairman of the Association of Police Authorities, agreed that authorities that oversee police forces "cannot afford to be complacent".
"Across a range of issues which could cause concern, the public will expect common sense to prevail and clear standards to be enforced with consistency," he said.
"We will work hard to implement the inspectorate's recommendations as quickly as possible and share best practice so that the public's overwhelming confidence in the police will deepen, not diminish, in the future.
"Looking ahead, HMIC is right to insist that it's essential that those who are elected to police the police are held to account as consistently as those they oversee if public trust in the service is to be maintained."
Paul McKeever, chairman of the Police Federation, which represents rank-and-file officers in England and Wales, said: "It is right that HMIC identify areas which the public consider could leave the police service vulnerable to accusations of corruption and that we put safeguards and tighter controls in place to ensure that confidence is maintained.
"The report recognises the importance of clear national standards.
"This is hugely important for the future integrity of the police service as we face an unprecedented level of structural changes in 2012 including the introduction of police and crime commissioners, whose priorities could be based upon populism and seeking re-election.
"This type of change could endanger the open and transparent nature of police governance and we would not wish to see the same accusations of corruption we have witnessed in the governance structure in the policing model in the USA mirrored here in the UK, with frontline police officers bearing the brunt of public frustration."