Commercial testing of DNA to predict potential health risks or establish ancestry could be inaccurate, harmful and a waste of money, experts warned yesterday.

The Human Genetics Commission, the UK Government's advisory body on human genetics, wants to see tighter controls introduced so people preparing to pay, in some cases, almost £100 are not misled and are aware of the implications.

There are a growing number of companies offering tests which they say will predict the risk of diseases, from bladder and breast cancer to arthritis and Alzheimer's.

Tests are also available which claim to be able to tell whether your ancestors were Picts, Vikings, or where in the world you originated.

However, the expert advice from the commission is: "Think carefully before spending your money - it could be a complete waste."

It is also warning that the medical tests could cause "unnecessary anxiety or give false reassurance". The ancestry tests may be perceived as harmless fun and bring pleasant surprises but they could come as a shock - revealing, for example, that a person's presumed mother or father is not their biological parent.

There is overwhelming support within the commission for a code of practice and when it met yesterday in Aberdeen it was agreed to work on a framework for the principles of direct genetic testing as a step towards a code.

Dr Christine Patch, genetic counselling manager at Guy's Hospital in London and a member of the commission, said commercial companies were testing for genetic variations which have been shown to be linked to diseases.

"Probably this development of testing is premature," she said. "If you had a family history of Alzheimer's disease and you were told you had an increased risk of getting Alzheimer's, that could be quite significant for you.

"The important thing for the person who buys that test is that they know what they are buying, that the information they are given is accurate and that the analysis has been carried out in a proper laboratory with proper controls.

"I think there is a risk that if people lose confidence in results and it becomes like a horoscope - it could lead into a loss of confidence in genetic science.

"I am not suggesting a ban but there needs to be standards. My personal opinion is these tests are a waste of money but people are curious about their own health and if an offer is made and it is not going to clearly harm them they should be allowed to take it up, but it doesn't tell them very much and it may tell them the wrong thing."

She said testing of children was a genuine cause for concern because the tests could be ordered online and sent through the post, making it impossible for laboratories to be confident that an adult who gives consent for the test actually has the authority to give that consent.

An adult could also pretend that a DNA sample taken from a child came from them. Professor Sarah Cunningham-Burley, a member of the commission from Edinburgh University, said: "The tests give only very limited information about one's ancestors and if you go back a few generations, the data is really not meaningful.

"The science of population movements is still itself in the quite early stages so that information isn't very robust.

"I would say think twice' before spending your money on that and think what information you really most want to know and if DNA analysis is actually going to provide it.

"There is potential for harm because people's identities are deeply rooted in who we are and information may challenge our assumptions about ourselves and our heritage."