"Implicit racial bias can occur even in people with a sincere belief in equality. Given the key role that such implicit attitudes appear to play in discrimination against other ethnic groups, and the widespread use of propranolol for medical purposes, our findings are also of considerable ethical interest."

Two groups of 18 participants took part in the study. Each volunteer was asked to undertake a "racial Implicit Association Test" (IAT) one to two hours after taking propranolol or the placebo.

The test involved categorising positive and negative words, and pictures of black and white individuals, on a computer screen.

Differences in the time taken to carry out the tasks provided the basis of the result.

More than a third of the volunteers had a "negative" IAT score, meaning they were biased towards being non-racist at a subconscious level. This was not seen in any member of the placebo group.

Propranolol had no effect on a different measure of "explicit" racial prejudice, religious and sexual prejudice, or prejudice against drug addicts.

These were tested using a "feeling thermometer" psychological tool used for assessing explicit prejudice. Volunteers were asked to rate how "warm" they felt towards different groups on a 10-point scale analogous to a thermometer.

The scientists wrote: "The main finding of our study is that propranolol significantly reduced implicit but not explicit racial bias."

Despite the study's small size and limitations, the researchers believe it raises important ethical and philosophical questions.

Co-author Professor Julian Savulescu, from Oxford University's Faculty of Philosophy, said: "Such research raises the tantalising possibility that our unconscious racial attitudes could be modulated using drugs, a possibility that requires careful ethical analysis.

"Biological research aiming to make people morally better has a dark history. And propranolol is not a pill to cure racism. But given that many people are already using drugs like propranolol which have 'moral' side effects, we at least need to better understand what these effects are."

But Dr. Chris Chambers, from the University of Cardiff's School of Psychology, said the results should be viewed with "extreme caution".

He said: "We don't know whether the drug influenced racial attitudes only or whether it altered implicit brain systems more generally.

"And we can't rule out the possibility that the effects were due to the drug incidentally reducing heart rate. So although interesting, in my view these preliminary results are a long way from suggesting that propranolol specifically influences racial attitudes."