A drug based on a compound in hallucinogenic mushrooms can improve the symptoms of severe depression for up to 12 weeks, a trial shows.

A 25mg tablet of psilocybin puts patients in a dreamlike state, making psychological therapy more likely to succeed.

But the short-term side-effects could be frightening and support must always be on hand, the researchers said.

An estimated 100 million people worldwide have serious clinical depression that does not respond to any available treatments - 30% attempt suicide.

Scientists have been studying the effects of psilocybin on mental-health disorders for years. Recent studies have been promising but too short to assess lasting effects.

In this latest trial, 1mg, 10mg and 25mg doses were tested on a total of 233 people from 10 countries in Europe and North America, with 25mg giving the best results.

consultant psychiatrist Dr James Rucker said the drug was thought to have "a direct action on the brain, putting it into a more flexible state and providing an window of opportunity for therapy".