More than 23,000 police officers and staff are moonlighting in second jobs, with the figure soaring nearly 20% in a year, it has been reported.
The figures mean that more than one in 10 officers in England and Wales earn a second income from non-police work, according to an investigation by the Mail on Sunday.
At the same time, the number of investigations into potential rule breaches has tripled, raising questions over conflicts of interest arising from second jobs.
Some officers may work in self-defence training, for example, meaning police forces may be commissioning off-duty staff to carry out such work for them, the article claimed.
A total of 23 of England and Wales's 43 forces did not check to see if they were paying companies run by their own officers for work.
Police staff are allowed to take second jobs or run companies if approved by their superiors. Unless there is a direct conflict of interest, permission is likely to be given.
The Mail on Sunday studied figures provided by Her Majesty's Inspectorate of Constabulary and found that at least 23,043 police staff had second jobs out of a workforce of 201,575 in May 2012. That was up 19% from March 2011 figures, which showed 19,329 had second jobs.
The number of investigations into second jobs soared in the nine months to May, with 154 reviews carried out – more than 17 a month. In the previous year, 82 – fewer than seven a month – were carried out.
Those investigations led to 10 officers leaving their jobs, either by being sacked or resigning, while 65 warnings were issued. Officers or staff were cleared of rule breaches in 35 cases while the other inquiries are ongoing.
A Home Office spokesman said: "It is the responsibility of chief constables to ensure that any secondary jobs held by officers do not harm the public's perception of the police or result in any conflict of interest.
"The home secretary will put new proposals ensuring the highest standards of integrity in the police to Parliament in the new year."
The news came as a senior Labour backbencher warned that public confidence in the police force has been shaken due to a "dangerous cocktail", including the "plebgate" affair and the results of the Hillsborough inquiry.
Keith Vaz MP, chair of the home affairs select committee, which will begin an inquiry into police accountability, integrity, internal corruption and malpractice next month, said it is a "defining moment" for the service.
He called on the prime minister to host annual summits with senior officers, and called for "a new Magna Carta" for policing.
In the Sunday Express, Vaz said recent events had dented the public's confidence in the police.
Writing about the alleged altercation between former chief whip Andrew Mitchell and police officers at the gates of Downing Street, dubbed "plebgate", Vaz said: "What appears to have happened to Andrew Mitchell could well have been a Christmas special script. The chief whip had to resign following a 60-second 'incident' in, of all places, Downing Street.
"Take a police officer apparently masquerading as a member of the public, a confidential log book finding its way into the public domain, add the results of the Hillsborough inquiry, which have resulted in thousands of serving and former police officers being investigated, and the fact that 26 out of the 43 police forces do not have a permanent chief constable, and you have a dangerous cocktail."
Vaz also pointed out that morale among officers is even lower as they feel they lack government support. He criticised the home secretary, Theresa May, for trying to enforce radical changes on the police force without having a proper dialogue with officers.
He acknowledged that existing police structures needed to be reformed, but said May's changes were "too rapid and too far-reaching".
In his article, Vaz rounded on the government for altering police officers' pay and conditions while trying to implement reforms. "One of the first rules of management is to ensure that during a period of radical change you carry your workforce with you. Unfortunately this has not happened," he wrote.
"With these profound changes taking place the last thing you should do is start to alter the pay and conditions of those who will implement the reforms without entering into a proper dialogue with them. The Government was wrong to change police pension arrangements retrospectively. It was unfair and forced out a number of experienced officers," he added.
Vaz's committee will next month launch an inquiry into issues of police training, accountability and integrity, and into the effectiveness of the processes for dealing with internal corruption and malpractice in the force.