The trial will not start until issues raised by the MHRA have been resolved between the regulator and the trial clinician, with talks with King's College London to take place this week.

A Department of Health and Social Care spokesperson said: ‘We have always been clear about the red lines regarding this trial – ensuring the safety and wellbeing of the children and young people involved and always being led by the clinical evidence.

‘The MHRA has now raised new concerns – directly related to the wellbeing of children and young people – and scientific dialogue will now follow with the trial sponsor.

‘As the evidence is now being interrogated by clinicians, preparations for the trial have been paused while the MHRA and clinical leaders work through these concerns.

‘This trial will only be allowed to go ahead if the expert scientific and clinical evidence and advice conclude it is both safe and necessary.

‘The safety and wellbeing of children and young people have always been the driving consideration in every decision we have made regarding this trial and always will be.'

An MHRA spokesperson commented: ‘With all complex clinical trials, MHRA's top priority is the safety and wellbeing of the trial participants.

‘It is part of the usual process that clinical trials are kept continuously under review and for us to have active scientific dialogue with the trial sponsors.

‘The safety and wellbeing of the participants to be recruited into the PATHWAYS clinical trial is paramount, particularly in view of the age of the children and young people who may be involved. For this reason, the MHRA is applying the highest scrutiny and taking a cautious and measured approach. We have raised some concerns related to the wellbeing of participants and scientific dialogue will now follow with the trial sponsor. We rely on the best scientific evidence to ensure all trials are as safe as possible for those participating.'

A letter from the MHRA to King's College London called for discussion of a minimum age for the trial suggesting 14 as the lower limit of eligibility due to the threat to fertility.

A spokesperson for King's College London said: ‘The wellbeing and health of young people with gender incongruence and their families has been, and will remain, our priority, and we will continue to work with the MHRA to support their further review of the trial, which has been designed by world-leading academics with scientific rigour at its core. That rigour and ongoing scientific discussion is important for any clinical trial, particularly one as complex as PATHWAYS, which aims to build an evidence base that can help young people and clinicians to make better-informed decisions in the future.'