The Prime Minister's spokesman sought to play down a BBC report that the Advisory Committee on the Misuse of Drugs (ACMD) had concluded that there was no need to re-classify cannabis again as a Class B drug.

He said that the report appeared to be based on a single presentation and that the committee had yet to reach any final conclusions.

Mr Brown, he said, stood by his comments at his Downing Street press conference on Tuesday when he said that the Government needed to send out a signal that cannabis use was not just illegal but also unacceptable.

"With regard to the Prime Minister's position, I think he made that fairly clear at his press conference," the spokesman said.

Mr Brown ordered the committee to carry out a review of the 2004 decision to downgrade cannabis to a Class C drug in one of his first acts on becoming Prime Minister last year.

According to the BBC report, the committee has now concluded there is no need re-classify it again after new research produced no evidence that rising cannabis use in the 1970s, 1980s and early 1990s had led to increases in the incidence of schizophrenia.

However, the Prime Minister's spokesman insisted that the committee had not made any final conclusions.

"Some of these reports seem to be getting ahead of themselves. We are not expecting to receive the report from the Advisory Committee on the Misuse of Drugs for at least another another month or so," he said.

"As we understand it, this report is based on one presentation that was given to a meeting. The advisory body themselves are still some way away from making any final conclusions on this."