Sir Keir Starmer is facing increasing rebellion from Labour MPs over plans to include puberty blockers for children in a recently launched medical trial.

Parliamentarians from the Prime Minister’s own party have demanded an end to the study, which could see around 226 young people recruited.

The youngest patients involved in the trial will typically be 10 to 11 for girls and 11 to 12 for boys, with a maximum age of consent at 15 years 11 months, researchers said.

One group will be given puberty blockers for two years, while the other will be given the drugs after a one-year delay.

The trial is being undertaken following a recommendation by the Cass Review into children’s gender care, which concluded that the quality of research claiming to show the benefits of such medication for youngsters with gender dysphoria was “poor”.

Several MPs, including Labour MP Jonathan Hinder, have written to Health Secretary Wes Streeting to voice their concerns.

Mr Hinder told The Sun: “When I was elected to Parliament last year, I knew that much of my party was still in the grip of this dangerous gender madness, but I thought the Government was starting to see sense.

“I now realise how wrong I was. I am working with a cross-party group of MPs, including Labour colleagues, to get this immoral experiment stopped.”

Another MP expressed concern about the possible long-term damage on those involved in the trial.

“The state shouldn’t be injecting 226 physically healthy eight to 13-year-olds to stop a natural biological process,” they told the newspaper.

Tory leader Kemi Badenoch also wants the trail to be abandoned immediately.

Despite mounting calls for it to be scrapped, Wes Streeting is still pressing ahead with the experiement.

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A source close to him said “it’s an important issue” but MPs are “right to debate, challenge and scrutinise”.

A Department of Health spokesman said it was following the recommendations of the Cass Review, which was accepted by the last Government.

It called for further evidence to be gathered through clinical research, while also recommending a ban on puberty blockers.

Puberty blockers are a drug used to delay the onset of puberty, often by blocking the production of oestrogen and testosterone.

Last year, the Government introduced a UK-wide indefinite ban on the drugs being prescribed privately or by the NHS to children and young people questioning their gender identity.

The new clinical trial, called Pathway, will involve children with a diagnosis of gender incongruence.

They will all have reached puberty, but will be younger than 16, and will have to meet a strict criteria.

Those undergoing the experiment will also have to undertake intensive medical and psychological screening before they are allowed to start taking puberty blockers.

Specialist NHS doctors involved with the trial must have a full picture of the young person’s wellbeing before making a decision as to whether they are suitable for the trial.

The young person will also have to demonstrate a good enough understanding of the potential impact of taking puberty blockers.

They will have to give their consent, along with the blessing of a parent or legal guardian.

Dr Hilary Cass said she was “really pleased” about the trial taking place.

She said: “My review uncovered a very weak evidence base for benefits from the use of puberty blockers for children and young people with gender dysphoria. In fact, some children had more negative than positive effects.

“However, given that there are clinicians, children and families who believe passionately in the beneficial effects, a trial was the only way forward to make sense of this.”

She added: “Although a lot of media attention has focused on the trial, they have a much broader research programme. Following the outcomes for young people who do not elect for a medical pathway will be just as important as following those who do.”