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."