Increasingly popular vaginal cosmetic surgery and genital piercing should in most cases not be prosecuted as female genital mutilation (FGM), according to fresh guidance issued by the Crown Prosecution Service.

In an attempt to improve the way police and prosecutors build cases in one of the most controversial areas of law enforcement, the CPS has revised its advice on what constitutes legitimate medical or therapeutic intervention and what amounts to criminal activity.

Since FGM was first outlawed in 1985, there has only been one conviction – earlier this year when a Ugandan woman, who lived in Walthamstow, east London, was sentenced to 11 years in prison for cutting her three-year-old daughter.

There have been several failed prosecutions, including the first in 2015 when a doctor was acquitted after stitching up a mother after childbirth. The offence has been a notoriously difficult legal area for prosecutors, with accusations that cases were pursued to satisfy political pressure for action.

The fashion for “designer vagina” procedures and genital piercings has complicated consideration of what is illegal and made clearer guidance necessary. Among the most popular types of operation is labiaplasty, which involves cutting labial tissue either side of the vulva.

Jaswant Narwal, who is the chief CPS prosecutor in Thames and Chiltern as well as the organisation’s lead on FGM cases, said: “Female genital mutilation is a sickening offence that can have a serious, lifelong physical and emotional impact on victims.

“We want to send a strong message that this crime does not have to be carried out in the UK for people to be prosecuted by the CPS – we will seek justice for people affected by this horrific practice. There is no hiding place.

“We hope this new guidance will give victims, police and prosecutors the confidence and practical guidance they need to bring more perpetrators of this traumatic abuse to justice.”

The Female Genital Mutilation Act 2003 increased the maximum penalty for FGM to 14 years. Suspects can face prosecution in the UK regardless of where in the world any procedure took place.

The young age of victims and a reluctance to report the crime to the police because of cultural taboos have both been blamed for low levels of offences being reported.

The new guidelines published on Thursday provide clarity on drawing the line between “designer vagina” operations and FGM, according to the CPS. In theory, some cosmetic vaginal surgery such as labiaplasty could fall under the definition of FGM in the 2003 act.

But prosecutors are advised they should also consider public interest factors, including the age of the alleged victim, whether they provided fully informed and free consent, the level of physical or mental harm caused, and the impact on the individual’s quality of life now and in the future.

Piercing female genitalia – “to adorn it with jewellery or other accessories purely for the purpose of personal decoration or in order to enhance the sensation of sexual contact” – will not usually amount to FGM, the advice says.

In relation to cosmetic surgery or labiaplasty, the guidance states: “Prosecutors should consider the evidence of necessity. This may include psychological reasons for the surgery which mean that the surgery will have a therapeutic element.”

Factors making it more likely that charges should be initiated are, among others, if the victim supports prosecution, if there is mental or physical harm caused, if the victim was under 18 and if there was no medical benefit.