The figures mean 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.
It comes as the Home Office released provisional figures to the Sunday Times which show crime has fallen by 10% in 19 out of 43 force areas in England and Wales between June 2010 and June 2012 despite budget cuts of just under 10%.
Crime has risen only in Devon and Cornwall, however the full impact of the cuts will not been seen across forces until April.
Damian Green, the police minister, told the Sunday Times that the statistics prove police reform is working, but the Police Federation chairman Steve Williams said the figures are testament to the hard work and dedication of police officers.
The Mail on Sunday report claims some officers may work in self-defence training, for example, therefore meaning police forces may be commissioning off-duty staff to carry out such work for them.
A total of 23 of England and Wales's 44 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 given. In 35 cases, the officers or staff were cleared of rule breaches and the other probes 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 Prime Minister David Cameron to host annual summits with senior officers, and called for "a new Magna Carta" for policing.
In the Sunday Express, Mr 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", Mr 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."
Mr Vaz also pointed out that morale among officers is even lower as they feel like they lack Government support.
He criticised Home Secretary Theresa May for trying to enforce radical changes on the police force without having a proper dialogue with officers.
He acknowledged existing police structures needed to be reformed, but said Mrs May's changes were "too rapid and too far-reaching".
In his article, Mr 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.
Mr 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.
PA
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