The number of unsolved rape cases have more than doubled in the past four years, according to data released by the Metropolitan Police.
Between 2016 and 2017, there were 164 unsolved rapes, compared with 78 between 2013 and 2014.
The number of unsolved sexual assaults also increased by more than 75 per cent in the same period, figures obtained under a Freedom of Information request reveal.
But the overall rate of unsolved crimes has not increased. Last year police recorded 22,209 unsolved crimes, only 464 more than four years before.
A police spokesperson pointed to a change in the way crime “outcomes” are recorded. As of 2014, Home Office guidelines replaced five possible “outcomes” – which were all logged as “detections” – with 19 possible categories of unsolved crime.
A spokesperson said: “For example, an offender to a specific crime may be known to police but the victim may no longer wish to support a prosecution.
“Pre-April 2014, this would have shown as an “undetected” crime. Using the new Counting Rules, this is classified as one of the 19 ‘outcomes’.”
He added: “Solving crime is a key priority for the Met and we are committed to doing all we can to cut crime, pursue offenders and support victims to make London even safer.”
This rise is partly explained by the rise in reported rapes in Islington, which have more than doubled since 2014, according to official figures.
There were 177 reports of rape between April 2016 and February 2017, and 90 between April 2012 and March 2013.
Rape cases also take longer to process than other crimes, according to a police report released in 2016.
And although a Crown Court case takes on average 171 days to complete, a rape case takes 247.
However, it is widely acknowledged that only a small proportion of the total number of rapes are reported to police.
A spokeswoman for Women Against Rape said: ““Although it’s good that women feel they can come forward now and aren’t going to tolerate what happened to them, unfortunately the conviction rate isn’t rising and we know there is a problem.
“We find that police aren’t taking rape seriously. They have an annual £36 million budget for PR, and while it’s good they are encouraging people to report, we find the police still can be biased and investigations aren’t done as thoroughly as they are meant to be.
She added: “From report to court, the prosecution often hasn’t gathered evidence properly.”