A seven-judge Supreme Court has dismissed a man’s appeal against his conviction for raping his mother on Mother’s Day.

The judgment provides guidance for judges in explaining to juries the law on rape, including in relation to defences based on alleged belief by an accused a woman consented to sex.

Belief that a woman “might” be consenting to sex is not a defence, the court stressed.

The man had claimed he genuinely believed his mother, aged in her 60s and with mobility problems, consented to sex. She was adamant she had not. He was convicted by a jury in 2012 and jailed for 12 and a half years.

In opposing his appeal, lawyers for the DPP had said it would be helpful for the Supreme Court to clarify there must be a genuine basis for consent in circumstances including where some hold views that a woman may be so “out of it” consent was irrelevant.

Giving the unanimous judgment, Mr Justice Peter Charleton said the incident happened in 2008 after the man called to his mother’s home. She was in bed having been out earlier with relatives for a Mother’s Day meal, got up, both consumed some “strong drink”, and started dancing.

She was also on some medication and told the jury she ended up on the floor with her son on top of her having sex with her. She said that was not consensual and she repeatedly told him to “leave me alone”. He maintained it was consensual.

Mr Justice Charleton said the crime of rape was “a terrible violation of a woman’s physical and mental integrity”.

The trial judge, in his charge to the jury, misstated the law by a mistaken reference that a belief by the accused a woman “might be consenting” rendered non-consensual sex somehow not a crime, he said.

A woman “has a constitutional right to her bodily integrity” and, as a matter of law, there must be a lack of consent by a woman. Sexual intercourse in such circumstances can only be excused if an accused honestly believed the woman was actually consenting, “not merely that she might be”, the judge said.

A proper consideration of the mental element in rape demonstrated the trial judge mistakenly stated the defence case in terms of law too widely but in favour of the accused, he said. As no error against the accused was identified, his appeal would be dismissed.

Earlier, the judge said the model chosen in the Criminal Law (Rape) Act 1981, as amended — which relates only to rape by a man of a woman — adopted not what a “reasonable man” believed concerning whether there was consent, but rather what an individual accused actually believed.

Addressing future directions in rape cases, he said those must necessarily depend on the particular elements of the prosecution and defence cases presented at trial. Trial judges need to also state no jury is “under any obligation to believe an obviously false story” and is entitled to accept or reject prosecution or defence evidence.

Where defences of mistaken belief in consent are advanced, every jury is entrusted to judge what the accused claims to be a mistaken belief against the view of what an ordinary or reasonable man would have realised in the circumstances. This defence requires “genuine belief”.

The definition of rape “is not at all dependent on force” and rape within marriage is also a crime, with some legal authorities indicating that was “always so”, even before the 1990 amendment of the Criminal Law (Rape) Act 1981.

“Lack of consent constitutes rape,” he said. Juries must decide if the external elements of the offence have been proven beyond reasonable doubt, the accused had sex with a woman who did not consent and, at the time, the accused knew she did not consent or was reckless whether she did or not.

Recklessness means the possibility a woman was not consenting actually occurred in the mind of an accused before they proceed or continue with intercourse. An honest, though unreasonable, belief a woman was consenting is a defence to rape but such alleged belief must be genuinely held.

There was a “stark divergence” between what the mother and son claimed and the jury’s verdict must be interpreted as dismissed the claims of the accused.