New anti-depressants have little clinical benefit for most patients, research suggests.

A University of Hull team concluded the drugs actively help only a small group of the most severely depressed.

Marjorie Wallace, head of the mental health charity Sane, said that if these results were confirmed they could be "very disturbing".

But the makers of Prozac and Seroxat, two of the commonest anti-depressants, said they disagreed with the findings.

A spokesman for GlaxoSmithKline, which makes Seroxat, said the study only looked at a "small subset of the total data available".

And Eli Lilly, which makes Prozac, said that "extensive scientific and medical experience has demonstrated it is an effective anti-depressant".

Patients are advised not to stop taking their medication without first consulting a doctor.

The researchers accept many people believe the drugs do work for them, but argue that could be a placebo effect - people feel better simply because they are taking a medication which they think will help them.

In total, the Hull team, who published their findings in the journal PLoS Medicine, reviewed data on 47 clinical trials.

They reviewed published clinical trial data, and unpublished data secured under Freedom of Information legislation.

They focused on drugs which work by increasing levels of the mood controlling chemical serotonin in the brain.

These included fluoxetine (Prozac) and paroxetine (Seroxat), from the class known as Selective Serotonin Reuptake Inhibitors (SSRIs), alongside another similar drug called venlafaxine (Efexor) - all commonly prescribed in the UK.

The number of prescriptions for anti-depressants hit a record high of more than 31 million in England in 2006 - even though official guidance stresses they should not be a first line treatment for mild depression.

There were 16.2m prescriptions for SSRIs alone.

The researchers found that even the positive effects seen on severely depressed patients were relatively small, and open to interpretation.

The seemingly good result was based more on the fact that the dummy placebo pills produced less of an effect in these patients, rather than on any notable positive response to the anti-depressants themselves.

Lead researcher Professor Irving Kirsch said: "The difference in improvement between patients taking placebos and patients taking anti-depressants is not very great.

"This means that depressed people can improve without chemical treatments.

"Given these results, there seems little reason to prescribe anti-depressant medication to any but the most severely depressed patients, unless alternative treatments have failed to provide a benefit."

Professor Kirsch said the findings called into question the current system of reporting drug trials.

Dr Tim Kendall, deputy director of the Royal College of Psychiatrists Research Unit, has published research concluding that drug companies tend only to publish research which shows their products in a good light.

He said the Hull findings undermined confidence in the ability to draw meaningful conclusions about the merit of drugs based on published data alone. He called for drug companies to be forced to publish all their data.

The National Institute for Health and Clinical Excellence (NICE) is currently reviewing its guidance on the use of antidepressants.

Marjorie Wallace of Sane commented: "If these results were upheld in further studies, they would be very disturbing.

"The newer anti-depressants were the great hope for the future.... These findings could remove what has been seen as a vital choice for thousands in treating what can be a life-threatening condition."

Dr Richard Tiner, of the Association of the British Pharmaceutical Industry, said there was no doubt that was a "considerable placebo effect" from anti-depressants when treating people with mild to moderate symptoms.But he said no medicine would get a licence without demonstrating it was better than a placebo.

Dr Tiner said: "These medicines have been licensed by a number of regulatory authorities around the world, who looking at all the evidence, have determined that they do work better than placebo."