POLICE are to issue their first ever warnings to men they suspect of sex crimes but cannot find enough evidence against to prosecute.

In a revolutionary new prevention tactic, officers will doorstep individuals who repeatedly come to their attention and who they believe - but cannot yet prove - pose a serious danger to women.

Senior Police Scotland officers hope such visits will have a chilling effect on persistently predatory men, in what they stress is a preventative move that does not mean they have given up ­criminal investigations.

Louise Raphael, the detective ­superintendent who leads Scotland's new national rape task force, said: "If there are people who we are not able to get a sufficiency of evidence against, because of the challenging nature of the crimes they are suspected of, then until now the only alternative we have had has been to do nothing.

"We either take them through the criminal process or we do nothing. Well, I don't think doing nothing is acceptable given we have a fundamental responsibility to prevent crime."

The new scheme - called Persons of Interest - will be launched shortly as a pilot in one of Scotland's 14 divisions. It will see officers visit such suspects and issue them with letters, but only on the authority of Ms Raphael or, in the future, another officer of her rank. The fact an individual has been targeted in such a way will not be shared with his employer or family, in a bid to stay on the right side of human rights legislation. Such suspects would almost certainly already have been spoken to by the police, often under caution while detained for questioning.

Police Scotland has overhauled ­attitudes to rape and sex offences since it came in to being 15 months ago - Ms Raphael is eager to tackle under­reporting, to see more victims come forward and to help build up the real picture of the crime. Nearly 1700 rapes were recorded in 2013-14, up one-quarter from a year before. Almost one-third took place more than 12 months before they were reported.

More reports, and information from other agencies and individuals on suspicious behaviour, can build up solid intelligence profiles.

She said: "We have been very keen to tap in to under-reporting of rapes. We can't encourage people to come forward to the police without putting in place a prevention strategy. We can't physically prevent sex crimes in the way we can physically prevent disorder. So we have to move our prevention upstream."

The Persons of Interest scheme is designed to chime with recent Police Scotland anti-rape messages, which are aimed firmly at raising the understanding of consent among men. Of particular concern are those who target drunk or otherwise vulnerable women who are incapable of agreeing to sex. "Some men might think they are just taking advantage of women who are drunk or have been separated from their friends on a night out," said Ms Raphael. "They need to realise that if a women can't consent they are committing rape."

Those likely to find the police at their door are the kind of ­predatory nightclub "sleazy" ­individuals for whom bouncers and bar staff are now trained to look out, sources said.

Similar warning tactics have already been put in place to deal with those whose behaviour with their partners has caused concern about domestic violence.

But QC Brian McConnachie, who has defended and prosecuted rape cases, is far from convinced by the tactic. "There is a risk that this could be abused," he said. "You know what communities are like, and word can quickly go round the police have been at somebody's door. The police should focus on gathering evidence for a prosecution. I should have thought interviewing somebody under caution would have a much more chilling effect than visiting them. So I don't see the point."

Sandy Brindley, of Rape Crisis Scotland, who has campaigned for better rape conviction rates, said: "Prevention should be a big part of what the police do."