Thousands of young American LGBT people will undergo conversion therapy from a licensed healthcare professional before they turn 18, a study has found.

An estimated 20,000 Teenagers aged 13 to 17 will go through an attempt to change their sexual orientation according to researchers at the Williams Institute at the UCLA School of Law.

Sometimes referred to as "reparative" therapy, counselling, prayer are used to try and "cure" people.

It has been widely discredited by health experts for risking causing emotional and psychological damage.

It is legal in the UK, but in January 14 health organisations - including NHS England and the Royal College of Psychiatrists - signed an agreement that described the treatment as "potentially harmful and unethical".

Conversion therapy is still legal in 41 states across the US, including Massachusetts and New York. Some state bans also apply to anyone who performs the practice in exchange for money.

The new study suggests an estimated 57,000 young people will receive the treatment from a religious or spiritual advisor.

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Researchers also found nearly 700,000 LGBT adults in the US have already received conversion therapy at some point in their lives, including 350,000 who were subjected to the practice as adolescents.

Lead study author Christy Mallory, state and local policy director at the Williams Institute, said many professional health associations and the public support ending the use of conversion therapy on LGBT youth.

“Our research shows that laws banning conversion therapy could protect tens of thousands of teens from what medical experts say is a harmful and ineffective practice,” she said.

According to the study, 6,000 youth ages 13 to 17 would have received conversion therapy before they reached adulthood if their state had not banned the practice.

None of them prevent religious or spiritual advisors from providing conversion therapy as long as they are acting solely in a spiritual capacity.

Several prominent professional health associations in the US, including the American Medical Association, the American Psychological Association and the American Academy of Pediatrics, have publicly opposed the use of conversion therapy.

Although talk therapy is a common technique, some practitioners have used “aversion treatments” such as inducing nausea, vomiting or applying electric shocks.