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Researchers want to hear from men who are forced by women to have sex against their will.

The study aims to uncover the extent of so-called "forced to penetrate" cases and has invited men who have experienced such circumstances to complete an online survey anonymously.

Examples could include a man waking up to find a woman having sex with him without his consent and a man being forced to have non-consensual sex with a woman because she is blackmailing him.

Another scenario could be a man being forced to have sex after being physically, emotionally or financially threatened.

Research leader Dr Siobhan Weare, of Lancaster University's Law School, said: "The term 'forced to penetrate' has been coined for these cases because, while they involve non-consensual sex, they do not fall under the offence of rape.

"The offence of rape can only be committed by men due to the requirement of penile penetration of the victim.

"In 'forced to penetrate' cases, the offender is the one being penetrated by a non-consenting victim.

"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.

"At the moment no data exists in this area the UK. While, anecdotally, 'forced to penetrate' cases exist, we have no means of measuring the experiences of men it happens to or the extent of the problem."

To complete the online survey, funded by Lancaster University, go to: http://wp.lancs.ac.uk/forced-to-penetrate-cases/