The number of blunders made at IVF clinics has nearly doubled in the past 12 months.
The serious mistakes, which affect couples desperate for children, include cases where embryos have been lost or placed in the wrong woman, or incidents where eggs have been fertilised with the wrong sperm.
Figures released by the IVF watchdog reveal the number of reported incidents increased from 182 in 2007-08 to 334 in 2008-09, prompting calls for it to get tougher on failing units.
The figures do not show which fertility clinics were the worst-performing, but include centres throughout England and Wales where 50,000 IVF procedures took place in the past year.
A leading embryologist said the Human Fertilisation and Embryology Authority (HFEA) was not adequately enforcing the rules on fertility treatment.
Sammy Lee, from University College London, said: 'I think the key failure of the HFEA is that when they ask clinics to put in special procedures, they're not enforcing them.
'It's important that when you've identified a weakness in a procedure, you quickly enforce it, and don't wait a year to do...