By Ben Hirschler, European Pharmaceuticals Correspondent

LONDON (Reuters) - Prozac and related antidepressants could in theory pose a cancer threat by blocking the body's innate ability to kill tumour cells, scientists have said.

But Professor John Gordon of the University of Birmingham, who led the research, said on Tuesday patients should keep taking their drugs since there was no evidence of any link in practice.

Working in the test-tube, Gordon and others found that the brain's mood-regulating chemical serotonin caused some cancer cells to self-destruct.

Eli Lilly's Prozac, Glaxo SmithKline's Paxil and Lundbeck's Celexa all "substantially blocked" this process.

The finding reopens controversy about the widespread use of the class of antidepressants called selective serotonin reuptake inhibitors (SSRIs) that first went on sale in the 1980s.

Millions of people with depression and anxiety have been prescribed the drugs, which have emerged as one of the biggest sellers for the international pharmaceutical industry. They work by stopping serotonin getting into cells.

Gordon's discovery that serotonin plays a role in killing a type of cancer called Burkitt's lymphoma was published in the online edition of the medical journal Blood.

"We've shown that, in the test-tube, the SSRIs stop the action of the serotonin on the cancer cells. But it's nigh on impossible to extrapolate to what's happening in the body," Gordon told Reuters.

"We must stress the effects shown for SSRIs on cancer cells is indirect and should cause no concern whatsoever to the many millions of people throughout the world who are prescribed this class of antidepressants."

A spokesman for the Department of Health said the research was at a very early stage and no increased risk of cancer had been detected.

Rather than being alarmed, Gordon is in fact excited that a new class of anti-cancer drugs may one day be developed that exploit serotonin's ability to kill cancer cells.

"Because we know the mechanism, we are now in a position to develop drug analogues of serotonin that will do the same job but have better pharmacological properties," Gordon said.

His work also provides an intriguing insight into the way that "positive thinking" associated with serotonin levels may play a key part in effective cancer care.

The mechanism by which serotonin can get inside cancer cells and tell them to commit suicide -- a process known as apoptosis -- suggests there is a clear "dialogue" between the brain and the immune system, he said.

Prozac was the first SSRI to reach the market in 1987 but it has since been overtaken by Paxil, also known as Seroxat, which racked up sales last year of 1.86 billion pounds.

Drug company officials said they did not believe their pills caused any increase in cancer and questioned whether the high doses used in Gordon's experiments may have affected the results.

"These data are from an in vitro (test tube) study and as such they cannot be extrapolated to a clinical setting with any degree of certainty," said Martin Sutton, a spokesman for GSK.