A trained phlebotomist at St George's Hospital, London, collects cord blood
Parents, hospitals and private firms are being warned over risky and illegal collections of umbilical cord blood.
The Human Tissue Authority said if proper guidelines were not followed samples may be contaminated and safety compromised on maternity wards.
The regulator has received reports of untrained and unlicensed people taking the blood - with one sample even obtained in a hospital car park.
The blood is sought because of its potential role in combating disease.
It is a rich source of stem cells, which are used to help treat diseases such as leukaemia.
CORD BLOOD COLLECTION
Samples can be collected while the placenta is still inside the mother or after it has been delivered
The umbilical cord is clamped and a tube inserted into it to drain away the blood
The sample is then put into cold storage and can be saved for years
More than 15,000 samples were taken last year, three quarters by private firms who charged parents a fee of up to £2,000
The therapeutic properties of cord blood first emerged in 1988
Since then more than 8,000 cord blood transplants have been performed worldwide, mostly to treat leukaemia
But some believe the potential of stem cells goes much further.
Stem cells can be used to create many different types of tissue and, in theory, could be used to repair and replace damaged organs as well as treating diseases ranging from Alzheimer's to cancer.
Due to the growing interest in the issue, the HTA started regulating cord blood collection in July 2008.
Since then its routine monitoring of sample collection has raised some worrying findings, according to the regulator.
It has received reports of more than 140 cases where samples were taken by people who were not properly qualified, but warned the figure was probably an underestimate.
These have included cases where parents have taken the blood themselves, medical staff have been pressured into doing it against their wishes or have done so unaware of the regulations.
Dr Shaun Griffin of 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."
The regulator is now writing to maternity units across the country as well as the organisations and individuals who are licensed to collect and store cord blood.
It has the power to prosecute people who are involved in unregulated sample taking, but has stressed at the moment it just wants to remind the NHS, industry and public of the rules.
Louise Silverton, general secretary of the Royal College of Midwives, reiterated the warning.
More than 15,000 samples were collected last year
"The time during the birth is one of the riskiest times in terms of safety. Therefore, 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."
The National Childbirth Trust also said it was concerned about the issue.
Last year more than 15,000 samples were taken across the UK. About a quarter of these were altruistic donations to the NHS Cord Blood Bank, charities or for research purposes.
The rest though are arranged privately with individuals paying firms up to £2,000 for collection and storage of the blood in the belief the blood could help the child if they get ill later in life.
However, many medical experts have questioned whether this is worthwhile as a person's own cord blood would be of little help in treating inherited diseases.
This page is best viewed in an up-to-date web browser with style sheets (CSS) enabled. While you will be able to view the content of this page in your current browser, you will not be able to get the full visual experience. Please consider upgrading your browser software or enabling style sheets (CSS) if you are able to do so.