The scientists from Imperial College London's Centre for Psychedelic Research measured participants' mood and functioning using a number of different measures.

Their primary measure scores people's symptoms of depression based on their answers to questions about sleep, energy, appetite, mood and suicidal thoughts.

These questions are largely negatively focused: they ask whether someone is feeling sad, but not whether they are feeling happy.

By this measure, psilocybin performed as well as a conventional antidepressant - an SSRI called escitalopram.

All 59 participants saw comparable reductions in their depressive symptoms.

But on several other measures - though they weren't the scientists' primary focus - the psychedelic drug performed considerably better.

That includes measures of work and social functioning, mental well-being and the ability to feel happy.

The study is among the first to pit the psychedelic head-to-head with a traditional depression treatment - and to open the trial to anyone with moderate-to-severe depression, not just those for whom all other treatments had failed.

Both groups were given therapy.

Prof Nutt said the therapy was "as important as the drug action" - the scientists are anxious to warn against people trying to self-medicate.

"It's not the drug alone. We're not sure the drug alone would have an intrinsic antidepressant effect," he said.