Patients will not start being recruited into the first clinical trial to examine the effects of puberty blocking drugs until early next year, months later than planned, NHS England has admitted.

The trial, which will study the “potential benefits and harms of puberty suppressing hormones for children and young people”, was originally due to start later this year.

Up to several thousand children and young people in England who are questioning their gender identity may opt to participate in the trial, sources said. The trial is being set up to bring forward evidence about puberty blockers after Dr Hilary Cass’s landmark review of transgender healthcare published in April said that they were an unproven treatment that could also harm people who take them.

The drugs are used to suppress the development of the sex hormones testosterone and oestrogen as well as the development of breasts and facial hair. Cass’s acclaimed report said the NHS should exercise “extreme caution” before giving them to young people.

The previous Conservative government banned their use and its Labour successor has backed that stance, with Wes Streeting, the health secretary, warning about the lack of evidence to show that they are safe to be used with such a vulnerable group of patients.

NHS England’s intention to enrol patients into a study that Cass recommended has prompted fresh controversy about the use of the drugs and how the health service should treat under-18s with gender incongruence.

Naomi Cunningham, the chair of the gender campaign group Sex Matters, said: “We understand why the NHS and Dr Hilary Cass believe that clinical trials of puberty blockers are necessary, but we would urge them to reconsider.

“Such trials are ethically unjustifiable, given the known risks of permanent damage to fertility, sexual functioning and general health.”

However, when NHS England announced in March that it would no longer prescribe the drugs, the trans rights charity Mermaids criticised the move as “deeply disappointing and a further restriction of support offered to trans children and young people through the HS, which is failing trans youth”.

The high court recently ruled that the ban on the use of puberty blockers in England, Scotland and Wales was lawful, rejecting a legal challenge by the advocacy group TransActual and a young person who could not be named.

In her judgment Mrs Justice Lang said: “The Cass review’s findings about the very substantial risks and very narrow benefits associated with the use of puberty blockers, and the recommendation that in future the NHS prescribing of puberty blockers to children and young people should only take place in a clinical trial, and not routinely, amounted to powerful scientific evidence in support of restrictions on the supply of puberty blockers on the grounds that they were potentially harmful.”

Critics, including the Good Law Project, have claimed that the NHS’s policy of restricting access to puberty blockers in recent years has led to more young people with gender dysphoria taking their lives. But a recent government-commissioned review rejected that claim.

In future all children and young people who are referred to what NHS England plans to be eight new regional gender services for under-18s will be told about the trial and asked if they want to participate.

Those services will replace the gender identity development service that was hosted until its disbandment at the end of March by the Tavistock and Portman NHS mental health trust in London. Two services are already operational and run by the children’s hospitals Great Ormond Street in London and Alder Hey in Liverpool.