Hospitals are facing an influx of drug users who have overdosed on MDMA – the main ingredient of ecstasy – because they thought they were snorting cocaine.

Professor David Nutt, incoming chairman of the Government's Advisory Council on the Misuse of Drugs, said drug users were buying white powder but did not know what it was.

"If you are buying white powders from someone, how do you know if you are getting MDMA, methamphetamine or cocaine? It's potentially very dangerous," he said.

The warning came at a meeting of the council in London as it heard from experts on the harm done by ecstasy. The council could recommend, when it reports next year, that the drug be downgraded from class A to class B. The Government is firmly opposed to the idea.

Dr Paul Dargan, head of the toxicology unit at Guy's and St Thomas' Hospital, said treating patients poisoned by MDMA powder they thought was cocaine was "a clinically common issue".