New figures reveal that police forces are writing off up to one third of all allegations reported to them

Official concern over a "culture of disbelief" in rape cases has been raised as new figures show that some police forces are writing off up to a third of all allegations reported to them.

A report by the high-powered joint government and police rape monitoring group confirms that a postcode lottery is operating in the way the 43 forces in England and Wales deal with rape allegations.

The figures show that the "no crime" rate for adult rape – the rate at which forces dismiss allegations initially recorded as a crime because of later details that emerge about the case – varies from only 3% in Cumbria to 33% in Lincolnshire.

Seven forces have a "no crime" rate for adult rape of 20% or more: Lincolnshire, Leicestershire (29%), Hertfordshire (21%) and the West Midlands (21%), Cleveland (20%), Derbyshire (20%) and Northumbria (20%). The national average "no crime" rate in adult rape cases is 12%, compared with 2% for all victim-based crime recorded by the police.

Dru Sharpling, HM Inspector of Constabulary (HMIC), who chairs the rape monitoring group, said the figures raised serious questions about whether a culture of disbelief persisted in some police forces when it comes to women making rape allegations. The police watchdog's recent report on the Jimmy Savile affair detailed the difficulties encountered by rape victims in going to the police, including lack of confidence, she said. She added that there might be a range of other explanations for the disparities between forces, but questions over disbelief had to be raised.

Rape Crisis of England and Wales said it was extremely concerned by the persistently high levels of "no criming" revealed by the data and the huge disparities between forces. The figures suggested there had been only limited improvement in police practice since a major report in February 2012. "This leaves us wondering how many more reports it will take before we see a marked improvement in criminal justice for rape survivors."

Liz Kelly, chair of End Violence Against Women, said: "Our member organisations know how deep disbelief and victim-blaming goes on in institutions and communities. But the police play a critical role enabling rape survivors to access justice, so these disparities and attitudes must be urgently tackled."

The report, published by HMIC on behalf of the rape monitoring group – which brings together senior police officers, crown prosecutors, law officers and officials from the Home Office and Ministry of Justice – collates all the official data on the problem for the first time. It comes six years after an official watchdog first warned that some reports of rape were being wrongly classified by police forces.

The report shows that despite a steady increase in the number of recorded rapes of adults and children, about 80% of those who are raped or sexually assaulted still do not report it to the police.

In the 12 months to March 2013 there were about 10,000 recorded rapes of adults in England and Wales, and about 6,000 recorded rapes of children. These figures have risen from 6,000 adult rapes and 5,000 child rapes recorded by the police in 2008-09.

Only 1,820 (18%) of those recorded rape allegations led to a "sanction detection" in which an offender was charged or cautioned for the offence, and 1,423 (12%) of cases were "no crimed" – which meant the original allegations were classified as no crime had happened.

In the Metropolitan police, where an internal review first established the scale of the problem in 2009, the "no crime" rate has nearly halved over the past five years to 13%. It now requires a senior officer to review all "no crime" decisions, which have to be at the standard of "beyond reasonable doubt". An external panel of academics will also carry out a quarterly review of all those allegations that have been dismissed in this way.

The figures show a postcode lottery in the rape detection rate. Durham police have the highest sanction detection rate at 32% but in Warwickshire it is only 6%. Northamptonshire had the highest rate of recorded rapes at 34.8 per 100,000 adults in the year to March 2013 while Durham had the lowest at 9.8. For child rapes, Humberside has the highest recorded rate at 106 per 100,000 children and Hertfordshire the lowest at 28. The average for the 43 forces is 59.5 per 100,000.

Jeff Farrar, of the Association of Chief Police Officers, said: "To build people's confidence in the way the police deal with sexual offences, it is my view that allegations of rape should be recorded as a crime when it is reported without question or challenge. This will help provide a consistent approach across the country that is supportive and victim-centred."

Martin Hewitt, the Met's deputy assistant commissioner, who is on the rape monitoring group, said it was a cause for concern that only about 20% of those who are raped or sexually assaulted report it. "This means that we are working from an incomplete picture of the offending and the majority of victims have no opportunity to access justice. As importantly, it means that rapists are avoiding arrest and conviction," he said. "One factor affecting reporting levels is the degree of confidence that victims have in the police response and how they believe they will be treated in the wider criminal justice system. It is clear from the data published today that there are inconsistencies between forces in the way that they deal with rape. This has the potential to affect victim confidence."

Helen Hopwood of the College of Policing said new guidance on investigating sexual offences would go out to consultation in April. "The datasets that have been published today highlight inconsistencies between forces about the outcome of rape investigations. This must be tackled."