Abuse of power is the worst of all forms of corruption, as Mike Cunningham, inspector of constabulary and former chief constable of Staffordshire police acknowledged in a BBC radio interview this morning . For the protector to turn abuser is, for the victim, an individual catastrophe – not least because she or he may never trust the police again. But the damage it causes to trust in wider society may, because of its wider implications, be even more grave. When a police officer abuses a victim of domestic violence, or any other vulnerable person, who has sought them out for help, it is an offence against the people the police exist in order to serve.

Earlier this year when Theresa May was still home secretary, she commissioned Her Majesty’s Inspectorate of Constabulary to find out how widespread such abuse is. The conclusions of the report that has just been released are shocking, but not surprising. In the two years to March, 436 allegations of abuse of power for sexual gain were made against 306 police officers, 20 police community support officers and eight staff. Yet only 40 officers or staff have been dismissed for abusing authority for sexual gain in a similar period. Forces appear to regard this kind of corruption inconsistently, with some treating it much more seriously than others. Even more than a generation later, it is as if there is still a lingering echo of the notorious canteen culture once identified in a mid-1980s report which portrayed police as racist and sexist, and not always entirely sober. More than a third of forces, according to the HMIC, need to improve their treatment of victims of domestic violence.

Yet it is four years since the Guardian first exposed the extent of police abuse of vulnerable women. In 2012, prompted by the conviction of a Northumbria police constable, Stephen Mitchell, who was jailed for life for carrying out sex attacks on vulnerable women while he was on duty, court records and disciplinary proceedings involving police across England and Wales were examined. They revealed 56 episodes ranging from rape to misconduct in public office. Some of the cases involved officers accessing the police national computer to gain details of vulnerable women to initiate sexual contact. Later that year, the Independent Police Complaints Commission concluded that referral of cases involving corruption had been inconsistent across police forces, and that chief constables must ensure that cases meeting the mandatory referral criteria were referred to the commission at an early stage.

Yet only last month, a freedom of information request to the IPCC showed that between 2012 and 2016, just 221 cases of abuse of power for sexual gain had been referred to the commission and, of those, only 50 resulted in a full investigation. That is very significantly less than the number of occurrences that the HMIC is now reporting. Once again, the IPCC’s chair, Dame Anne Owers, has dispatched a letter to all chief constables to reinforce the message that abuse constitutes very serious corruption and must always be reported. As a spokesperson admitted, “we don’t know what we don’t know”. So it is all the more welcome, if belated, that the police minister, Brandon Lewis is considering changing the police regulations in order to make referrals of all police abuse cases mandatory.

The same inconsistency haunts the figures for domestic violence; but it seems beyond doubt that the extent is growing. The latest official statistics show that it accounts for a third of violent crime and one in 10 of all recorded crimes – more than a million cases. The great majority of the attacks are by men against women. Earlier this week, the Femicide Census showed that between 2009 and 2015, 936 women were killed by men in England and Wales, and that most women killed were killed by a man known to them.

Mrs May probably did more than any of her predecessors to improve the way domestic violence is handled in her six years as home secretary. She strengthened and extended the law, and demanded that the police treat it more seriously and more consistently. Two years ago, she extracted an extra £40m of funding from the Treasury for women’s refuges. But when councils are so short of cash to meet their statutory obligations, the women’s aid organisations doubt it will do more than patch up parts of a service that has been hard hit over the past five years and which sorely needs reliable funding. Violence against women is an underrated offence that exposes at best a failure to understand, and at its worst, the enduring nature of institutional misogyny.