Plans to sell genetic tests over the internet so people can find out if they are at risk of developing mental illnesses have been denounced by leading UK psychiatrists. They say the technology is still primitive and is only likely to worsen individuals' emotional and mental problems.

The tests, which biotechnology companies will begin selling in a few months, will allow people to find out, by sending off a spittle sample, if they possess gene variants that increase their chances of suffering bipolar depression or schizophrenia. The information will help both patients and doctors, it is claimed.

But scientists argue that selling these tests on the internet is dangerous. The technology is still in its infancy and cannot yet help make helpful diagnoses.

'These tests will only worry, confuse and mislead the public and patients,' said psychiatrist Professor Nick Craddock, of Cardiff University. 'There is a long way to go before we have genetic tests that may be helpful to patients. Using tests at the moment is only likely to cause harm.'

This point was backed by Professor David Collier, of the Institute of Psychiatry, London. 'At best, these tests are clinically useless. At worst, their results could cause serious worries for patients.'

The tests are based on recent discoveries of gene variants that can increase a person's chance of succumbing to depression or schizophrenia. Each variant raises a person's risks by only a slight margin but if tests for several variants are combined, this creates a useful risk assessment for an individual, firms say.

As a result, several - including NeuroMark in Colorado, Psynomics in California, and SureGene in Kentucky - have developed tests based on these variants and are preparing to sell them for several hundred pounds a test on the internet.

For example, Psynomics has developed a test for pinpointing individuals at risk of depression. 'This is going to be initially driven by patients,' John Kelsoe, chief executive, told Science magazine last week.

But the usefulness of such tests was disputed by Dr Cathryn Lewis, also of the Institute of Psychiatry. 'The general risk of developing bipolar depression is around one per cent. If you possess the worst set of gene variants, then your risk rises to three per cent. That means you are three times more likely than average to get bipolar depression. That may seem worrying but it is still a very low risk. It is still 97 per cent likely that you won't get depression. People are not likely to realise that, however.'

Another test - to be marketed by NeuroMark, first in the US and later this year in Europe - is based on genes that predispose people to react badly to stress. If a person inherits this gene section from both parents, he or she has an increased chance of suffering from severe depression after stressful situations. 'About 20 per cent of people have this combination,' said Kim Bechthold, chief executive of the biotechnology company. 'It is useful information to know.'

But if a person proves positive, what action can they take to avoid risk and counter the risk they face? 'We suggest joining a choir, eating chocolate or buying a puppy,' said Bechthold.

These ideas were dismissed by Craddock. 'Life is stressful. You cannot avoid it. It is extremely unhelpful to tell people that they face a risk like that.'

In addition, scientists have found only a small number of risk gene variants for psychiatric disorders. Many more await detection, it is thought. 'The trouble is that if someone takes a test which suggests they face no risk, they could easily think they can then do what they want,' added Craddock. 'They might start taking recreational drugs again, which would pose problems if they are prone to depression. Other risk gene variants might later be discovered and new tests developed. Then we will find these people actually have an inherent proneness to depression after all. It will be too late by then, however.'