The Human Tissue Authority (HTA) has warned that some organisations which collect umbilical cord blood to preserve stem cells could be breaking the law.
It follows recent concerns that parents are being asked to collect the blood themselves using kits delivered to their homes.
But the HTA has written to more than 150 organisations, including maternity units and private firms, to remind them of their duties.
According to HTA data, there were 15,514 cord blood collections last year, an increase from the 14,335 carried out in 2008.
Private companies charge around £1,500 for extracting stem cells from the blood and storing them for up to 25 years.
Cord blood is a rich source of stem cells, which have the potential to grow into many different kinds of body tissue.
The HTA said it had written to maternity units, firms and professional bodies to warn that unlawful umbilical cord blood collection may compromise patient safety and quality of care.
Dr Shaun Griffin, Director of Communications at the HTA, said: "Collection of cord blood is the same as any other medical procedure: it needs to be carried out safely by trained staff because collection is not without risk to the mother and baby."
Louise Silverton, General Secretary of the Royal College of Midwives, said: "It is essential that midwives are able to concentrate on the birth and are not put under pressure to carry out unregulated and unlawful cord blood collections.
"If parents are thinking about cord blood banking, they need to discuss this with their midwife, or other health professional, to find out if collection is viable."