When police officers who rape, sexually assault or harass vulnerable victims do reach the court system, phrases such as abuse of power and breach of trust ring out from the judicial benches.

Judges and prosecutors have pronounced over the past four years upon the "rarity and exceptional nature" of these crimes by police officers. A senior crown prosecutor expressed that view, for instance, last year in connection with Mark Wilkie, a Bedfordshire constable who was jailed for 40 months last August for trawling the police computer system to access details of victims of domestic violence and teenage runaways.

The then 51-year-old officer sent the women and young girls hundreds of menacing and sexually explicit messages – adding a threat that he would be watching them.

Wilkie pleaded guilty at Cambridge crown court to 12 counts of misconduct in public office and one count of theft.

Bedfordshire constable Mark Wilkie sent explicit text messages to vulnerable women after illegally retrieving their details from the police database. Photograph: SWNS.com

But despite repeated claims that these cases are rare, the true scale of the abuse is not known. There has been little or no research carried out and the police service seems reluctant to address the issue publicly.

The 56 cases documented today by the Guardian from 2008 to 2012 involve 25 forces, 48 officers from the rank of constable up to deputy chief constable, five police community support officers, one special constable, one civilian police officer and one member of police staff.

They have been found variously to have abused their positions to rape, sexually assault, harass, groom and have inappropriate relationships with vulnerable women and young girls, or have been investigated over such allegations. Others are awaiting trial for alleged offences.

The cases show similar themes: the ease with which police officers can access and misuse confidential information, a lack of supervision and awareness of suspect behaviour, a failure to record and monitor the history of complaints, and a trend in which women who complain they have been raped, sexually assaulted or inappropriately targeted by a police officer have to battle to be believed.

The victims run into the hundreds and include female police staff, vulnerable women who have been victims of crime and young people.

Professor Jennifer Brown, co-director of the Mannheim centre for criminology at the London School of Economics, carried out landmark research 19 years ago that revealed 800 policewomen had claimed they had been raped or subjected to a serious sexual assault by male colleagues. The Home Office never published the study.

Brown said the issue of abuse of power by officers for sexual reasons was not well documented. "The questions that need asking are, why hasn't this been looked at and how prevalent is it?" she said. "If the Guardian has uncovered more than 50 cases of this kind of abuse of authority, one could imagine there will be many more which are not accessible through public documents."

In several cases examined by the Guardian, the Crown Prosecution Service chose not to prosecute. When a young woman who had been attending a party in Russell Square, central London, complained in 2010 that she had been lured to a police locker room in St Pancras station and raped by a Metropolitan police officer, prosecutors did not lay charges against the officer. However, the officer, who was never identified by the force, was later sacked after an internal misconduct panel found that he had intentionally had sex with the woman, and she had not consented.

The campaign group Women Against Rape, which supports victims of predatory police officers, is critical of the decision not to pursue criminal charges.

Among the vulnerable women who have been targeted are victims of domestic violence. James Formby, a then 31-year-old Metropolitan police constable, was called to a report of a domestic disturbance in September 2009 with colleagues. While an officer removed the man from the house, Formby remained at the scene to take a statement, Southwark crown court heard. But in the process he began flirting with the woman, who had been drinking and had taken drugs. He ended up in the bedroom with the woman, where the court heard that a "sex act" took place.

In 2010, Formby was given a 20-week jail sentence suspended for two years and dismissed from the force after admitting misconduct in public office. After his sentence his victim condemned the leniency of the term.

A Greater Manchester police constable, Michael Fletcher, then 31, was jailed last year for 32 months over similar behaviour with a victim of domestic violence. Fletcher – who admitted misconduct in public office at Manchester Minshull Street crown court – had sex twice with a woman in July 2010 after responding to a welfare call to her home.

Ten victims of the Northumbria police constable Stephen Mitchell, 43, who was jailed for life in January 2011 for raping and sexually assaulting women he met on duty, are now seeking compensation from the force. Northumbria police said it had paid out £31,500 to some of the women and was still considering outstanding claims.

Mitchell's victims were mostly vulnerable heroin addicts and sex workers, and he told one of them that if she complained "no one would believe a junkie".

Mitchell's case revealed failures within the service to monitor and act upon previous incidents of suspicious behaviour, including an incident in which he was disciplined for having sex with a woman he met on duty and accessing confidential information on the force computer.

It also exposed vetting weaknesses – Mitchell had been charged with a serious sexual assault before he joined the police, which was never uncovered.

The Northumbria force told the Guardian that since the Mitchell case, it had toughened up its own vetting and disciplinary processes and had advised other forces on the same issues.

Derbyshire police turned to Northumbria for help during an investigation into one of their constables, Jasbir Dhanda, two years ago. Like Mitchell, Dhanda, then 52, was accused of targeting vulnerable women and sex workers and using his position as an officer to make them have sex with him.

Northumbria constable Jasbir Dhanda was jailed after targeting sex workers and vulnerable women and using his position to make them have sex with him. Photograph: Caters News Agency

He was jailed for two and a half years in January for having sex with one sex worker while on duty and using the police database to search for personal information about other women he intended to target. Dhanda was convicted of three counts of misconduct in public office and three counts of obtaining personal data using a police database.

At Nottingham crown court, Judge Michael Stokes QC told him: "You were taking advantage of a woman, sexually, who you knew … was highly vulnerable, had real difficulties in her life and was someone, if she complained, who was unlikely to be taken seriously."

His words mirror the concerns of those who work with victims of police officers and those of the Independent Police Complaints Commission (IPCC), which highlighted in its report on police corruption last month that "the credibility or background of some of the victims [eg some were sex workers] may have meant that initial allegations were called into question".

The IPCCis likely to recommend in its abuse of power report later this year that forces make more use of covert techniques to investigate and gather evidence about officers' behaviour to corroborate the accounts of alleged victims.

Such tactics were used last year in Cumbria, where Detective Constable Mark Fisher, then 49, – nicknamed Fish the Flirt by his colleagues – was jailed for four years after being found guilty of accessing confidential information on women who had been victims of crime in order to initiate sexual relationships with them.

The force's department of professional standards mounted a covert investigation after concerns were raised about his behaviour. Fisher was found guilty of five counts of misconduct in public office.

The IPCC will also examine failures to record and monitor disciplinary offences during an officer's career, and whether more supervision is needed in this area.

The IPCC commissioner, Amerdeep Somal, said: "Police personnel who abuse their position and exploit women for sexual gratification have no place in policing. The IPCC has investigated some cases of serious concern."

But the police complaints procedure, of which the IPCC is part, is mistrusted by some groups who work with victims of predatory police officers. Lisa Longstaff, from Women Against Rape, said the whole issue of sexually predatory police officers was veiled in secrecy. "Policewomen and police staff, wives of officers as well as women civilians, have disclosed to us rape and sexual assaults by officers which never went to court," she said.

"Much illegality and violence by officers is dealt with internally and away from the public, and is often not put on their record so no one outside the force knows about it and they are allowed to continue working their way up the ranks."

Inspector Kate Pain – the Police Federation chair for the Wiltshire force, where Deputy Chief Constable David Ainsworth killed himself last year during an investigation into 26 claims of sexual harassment against him from 13 female staff – is one of the few to speak openly about a pervading culture of sexism in the service.

In the Ainsworth case, two female staff who made the original complaints have received a settlement and an apology from the force. But they are now the subject of an investigation by an external force being managed by the IPCC – something their union, Unison, says is an appalling way to treat "whistleblowers".

Pain questioned whether the whistleblowing system worked within the police service. "The police is still male-dominated and it is very difficult for people to come forward to challenge poor behaviour and performance because they are worried about promotion to the next rank, they are worried about not being supported by their boss and they are worried about being isolated," she said.

"That was exacerbated in the extreme when the individual was the second most influential officer in the force. What we need is a greater weight of numbers of women in higher ranks to challenge the male dominated approach to policing.

"Tackling racism was forced on the police service, we had no choice post-Stephen Lawrence and the scrutiny and questions around institutionalised behaviour. We have not had that with profound sexism within the police service. I think we need to open up the culture and look at it."