Men who have been forced by women to have sex against their will are being urged to take part in a new study investigating the "hidden crime".

So-called "forced to penetrate" crimes are not currently legally classed as rape - but researchers say they know such cases exist and need to be brought to light.

Dr Siobhan Weare, from Lancaster University's Law School, is leading the study as she said there is no official data about men who are forced to penetrate.

The study states: "Whilst there is a wealth of research regarding women’s non-consensual sexual experiences with men, and a growing body of research in relation to men’s non-consensual sexual experiences with other men, there is no research in the UK on men who are ‘forced to penetrate’ women."

This is a hidden crime. The findings of this research will enable a greater understanding of such experiences and will help to develop practice and policy in this area, as well as in relation to the broader issue of men who experience sexual violence. Dr Siobhan Weare, Lancaster University

Legally, the offence of rape can only be committed by men as it relates to a victim being penetrated by a penis vaginally, orally or anally.

But in 'forced to penetrate' cases, the offender is the one being penetrated.Dr Weare said examples could include men who have woken up to find a woman having sex with him without his consent.

Other scenarios could be a man being forced to have sex because she is blackmailing him, or after being physically, emotionally or financially threatened.

Anyone who would like to take part in the study can fill in an online survey.