People who change their gender often end up with horrendous results or are left feeling damaged, Lord Robert Winston has said.
The British scientist and TV presenter warned the implications of gender reassignment surgery were not properly understood, with complications that could affect fertility.
On BBC Radio 4's Today programme, he said "results are horrendous in a big proportion of cases".
He said 40 per cent of those who undergo vaginal reconstruction surgery experienced complications requiring further surgery, and 23 per cent who had breasts removed felt "uncomfortable with what they've done".
"What I've been seeing in a fertility clinic are the long-term results of often very unhappy people who now feel quite badly damaged."
The effects of taking hormones were "likely to affect reproductive function", he said.
Professor Gary Butler, an expert in care for transgender children and teenagers, agreed that the effect on fertility was a "worry".
The consultant in paediatric and adolescent medicine at University College Hospital in London also agreed that doctors had to be "very careful" about introducing therapy before puberty.
Experts have raised concerns about the increasing number of children who question their gender identity after figures showed the total had increased dramatically in five years.
Children who want to transition are not offered surgery but can be given synthetic hormones to slow puberty.
But campaigners argue those not allowed to transition risk developing serious mental health issues.
Figures published earlier this year by Stonewall, an LGBT charity, suggested half of young transgender people had attempted suicide.