Sexual predators in the police are abusing their power to target victims of crime they are supposed to be helping, as well as fellow officers and female staff, the Guardian can reveal.

An investigation into the scale and extent of the problem suggests sexual misconduct could be more widespread than previously believed.

The situation raises questions about the efficacy of the police complaints system, the police's internal whistleblowing procedures, the vetting of officers and a failure to monitor disciplinary offences.

Police officers have been convicted or disciplined for a range of offences from rape and sexual assault to misconduct in public office relating to inappropriate sexual behaviour with vulnerable women they have met on duty. Others are awaiting trial for alleged offences, though many are never charged with a criminal offence and are dealt with via internal disciplinary procedures.

The problem is to a large extent hidden, as no official statistics are kept and few details are released about internal disciplinary action in such cases.

By analysing the data available – including court cases and misconduct proceedings – the Guardian has attempted to document the scale of the corruption for the first time.

In the past four years, there were 56 cases involving police officers and a handful of community support officers who either were found to have abused their position to rape, sexually assault or harass women and young people or were investigated over such allegations.

The Independent Police Complaints Commission (IPCC) and the Association of Chief Police Officers (Acpo) are so concerned they are carrying out a rare joint inquiry into the scale of the problem, which will be published in September, the Guardian can reveal.

Their work was prompted by the case of the Northumbria police constable Stephen Mitchell, 43, who was jailed for life in January 2011 for carrying out sex attacks on vulnerable women, including prostitutes and heroin addicts, while he was on duty.

Despite being the subject of previous disciplinary offences, involving one inappropriate relationship with a woman and the accessing of the force computer to find private details of an individual, Mitchell had not been subjected to extra supervision or dismissed by the force.

Those targeted by the officers are predominantly women, but in some cases are children and young people, many of them vulnerable victims of crime.

The Guardian's investigation has uncovered evidence of:

• Vetting failures, including a concern that vetting procedures may have been relaxed post-2001 during a surge in police recruitment.

• Concerns over the recording and monitoring of disciplinary offences as officers progress through their career.

• A tendency for women who complain they have been sexually attacked by a policeman not to be believed.

• A pervasive culture of sexism within the police service, which some claim allows abusive behaviour to go unchecked.

Debaleena Dasgupta, a lawyer who has represented women sexually assaulted and raped by police officers, said: "I don't think any [victims] are quite as damaged as those who are victims of police officers.

"The damage is far deeper because they trusted the police and … believed that the police were supposed to protect them from harm and help catch and punish those who perpetrate it.

"The breach of that trust has an enormous effect: they feel that if they can't trust a police officer, who can they trust? They lose their confidence in everyone, even those in authority. It is one of the worst crimes that can be committed and when committed by an officer, becomes one of the greatest abuses of power."

The officers involved come from all ranks within the service: the most senior officer accused of serious sexual harassment was a deputy chief constable, who was subject to 26 complaints by 13 female police staff.

David Ainsworth, deputy chief constable of Wiltshire police, killed himself last year, an inquest heard this month, during an inquiry into his behaviour. He is one of two officers accused of sexual misconduct to have taken their own lives over the past four years.

In one of the worst cases in the past four years, Trevor Gray, a detective sergeant with Nottinghamshire police, broke into the home of a woman he met on a date and raped her while her young child slept in the house. Gray was jailed for eight years in May for rape, attempted rape and sexual assault.

Many of the cases documented involve police officers accessing the police national computer to gain access to the details of vulnerable women and young people in order to bombard them with texts and phone calls and initiate sexual contact.

Deputy Chief Constable Bernard Lawson of Merseyside police, the Acpo lead on counter-corruption, who is working with the IPCC on the joint report, said: "Police officers who abuse their position of trust have an incredibly damaging impact on community confidence in the service.

"There is a determination throughout policing to identify and remove those who betray the reputation of the overwhelming majority of officers."

In its report on corruption within the police service published last month, the IPCC identified abuse of authority by officers for their own personal gain, including to engage in sexual intercourse with a vulnerable female while on duty, and the misuse of computer systems to access details of vulnerable females, as two of the five key corruption threats to the service.

IPCC figures show that 15% of the 837 corruption cases referred by forces to the watchdog between 2008 and 2011 involved abuse of authority by a police officer, and 9% involved misuse of systems.

Clare Phillipson, director of Wearside Women in Need, who supported some of Mitchell's victims, said: "What you have here is the untouched tip of an iceberg in terms of sexually questionable behaviour and attitudes. The police service, in my experience, has an incredibly macho culture and women are seen as sexual objects.

"Police officers have a duty to steer away from vulnerable women in distress, some of whom see these police officers as their saviours. It is an abuse of their power to exploit that."

One area to be examined by the IPCC is whether there might have been vetting failures from 2001 onwards during a massive recruitment drive in the police.

Between 2001 and 2007, the overall strength of the service grew by more than 16,000, with around 2,666 officers recruited each year on average.

Six years ago, a study of vetting within the police service by Her Majesty's Inspectorate of Constabulary revealed "disturbing" failures that had allowed suspect individuals to join the service. The report, Raising the Standard, exposed more than 40 vetting failures among police officers and support staff. The report concluded: "The potential damage that can be caused by just one failure should not be underestimated."