Porn star Ron Jeremy has "severe dementia" and will be declared unfit to stand trial for rape, a US newspaper reported Friday.

Jeremy, who has appeared in more than 1,700 X-rated films since the 1970s, was ordered to undergo psychiatric exams ahead of a trial on multiple sex charges against 21 women and girls.

The Los Angeles Times, citing an email it has obtained, reported mental health experts for both prosecution and defense have found he is mentally incapacitated, with little chance of recovery.

"As a result of the agreement of the experts, the defendant will be declared incompetent to stand trial... his prognosis for improvement is not good," Los Angeles County deputy district attorney Paul Thompson wrote in the email, the paper said.

"If he does not improve, we will not be able to try him for his crimes. Because criminal proceedings are suspended as long as he is incompetent, we also cannot get a guilty plea from him or discuss other measures to get justice for the victims in this case."

Jeremy was initially charged in January 2020, and prosecutors subsequently added a battery of other sex-related complaints against him, alleging criminality over several decades of his prolific career.

Jeremy has previously denied all charges against him.

Judge George Lomeli ordered a psychiatric assessment after one of Jeremy's attorneys, Stuart Goldfarb, said he had visited his client in a holding cell before the hearing.

"He was unable to determine who I was," Goldfarb said.

A declaration of mental incompetency is not set in stone, and if a defendant's health improves through medication or treatment, they can face trial at a later date.

However, dementia is a progressive illness, and sufferers generally degenerate over time.

Born Ronald Jeremy Hyatt, the performer appeared in adult movies such as "Deep Throat II" and "John Wayne Bobbitt Uncut."

With his trademark moustache, he was one of the industry's most recognizable stars among the wider public.

But he has long been dogged by assault claims and has been barred from industry events in recent years.

After the initial charges were brought against Jeremy in 2020, Goldfarb denied all the allegations and insisted his client was "not a rapist."

"Ron -- over the years, because of who he is -- has essentially been a paramour to over 4,000 women. And to allege that he is a rapist is beyond... I mean, women throw themselves at him," he added.