Suspects are increasingly being left in legal limbo as they are subjected to inquiries lasting years that sometimes end with the case being dropped, a survey of criminal solicitors has revealed.

The full extent of police use of the “released under investigation” (RUI) tactic is revealed in a study that shows more than half of firms who responded represent clients who have been waiting for between 18 months and two years.

The study by the London Criminal Courts Solicitors’ Association (LCCSA) was conducted in the capital, the south-east and north-west of England. In one case a suspect was found to have been waiting four years for his case to be resolved.

A total of 6,519 cases designated RUI were reported in the past three months by the 109 criminal lawyers who responded to the survey. The alleged offences included serious crimes including rape, sexual assault, homicide, fraud, bribery, theft, racially aggravated crimes and dishonesty.

Of the lawyers who said clients had been under investigation for between 18 months and two years, most reported the cases involved allegations of rape, sexual assault and serious drug offences – leaving victims and suspects without safeguards or supervision.

The Policing and Crime Act 2017 established tighter conditions for police bail, limiting it to 28 days with three-month extensions on the authorisation of a senior officer. It was introduced after the high-profile case against the BBC presenter Paul Gambaccini, who campaigned to end unlimited bail: the case was eventually dropped.

The complexity of the new regulations and short deadlines, however, mean that officers are increasingly failing to subject suspects to formal conditions such as reporting regularly to police stations.

In one example highlighted by the LCCSA, a suspect was released under investigation for rape. His partner left him, he lost his home and business, and at times felt suicidal. “I’ve been to hell and back, living under a cloud of suspicion for more than two years,” Duncan (not his real name) said. Just over two years after arrest the case against him was dropped.

Kerry Hudson, a defence solicitor and the vice-president of the LCCSA, said: “The legal limbo of releasing suspects under investigation came about as an unintended consequence of a law change to arrest and charging laws.

“Once a 28-day time limit was imposed to bail it became obvious the police could never keep to that timeframe. This legal no man’s land emerged as a fudged workaround by hard-pressed, under-resourced police.

“It is failing everyone in the justice system. For suspects – who, let’s remember, are innocent until proven guilty – it spells a life on hold, causing untold stress and mental health problems, shattering families, homes and livelihoods. We’re very aware it lets victims and complainants down too.”

Rumit Shah of the London law firm Galbraith Branley said: “Most clients find it difficult to move on with their lives. Some are left in limbo for such a long time that it has an impact on their mental wellbeing and affects the loved ones around them. With youths they struggle to continue their education because they constantly have the investigation in the back of their mind.”

A Home Office spokesman said: “The police can still use pre-charge bail in cases where it is necessary and proportionate, including where bail conditions are needed to protect victims.

“The National Police Chief’s Council guidance, issued to police forces earlier this year, states that in cases involving high-harm crimes, such as domestic abuse and sexual violence, pre-charge bail should be seriously considered and senior detectives consulted if a suspect is released only under investigation.

“Cases where individuals are released under investigation must also be regularly reviewed and effectively managed, with both suspects and victims kept updated.”