By Michael Holden

LONDON (Reuters Life!) - Britain's most senior police officer said on Friday that pop stars and celebrities who are pictured apparently taking drugs should be put on trial to prove they were snorting talcum powder and not cocaine.

London Metropolitan Police Commissioner Ian Blair said he wanted juries to decide whether famous figures caught on film were indeed taking illegal drugs even if it was impossible for police to prove what the substance actually was.

A number of well-known stars such as supermodel Kate Moss and Grammy-winning soul singer Amy Winehouse, have been pictured in national newspapers allegedly taking drugs, prompting police investigations that led to no charges.

In an interview with the London Evening Standard, he said he had "directly asked" the Crown Prosecution Service (CPS) whether it was reasonable celebrities should go on trial when "we can't tell what kind of white powder that is".

"My position is that a sensible jury would not expect people to be sniffing talcum powder," he said in the interview, a transcript of which was made available by police.

Blair's comments come after his force carried out a nine-month investigation into allegations that Moss had taken illegal drugs.

Tabloid newspaper the Daily Mirror published photographs in June 2005 which showed Moss apparently snorting large quantities of cocaine. But the following June, prosecutors decided there was insufficient evidence for criminal action.

The CPS said analysis of the film of Moss had narrowed the substance she was taking down to one of three drugs -- cocaine, ecstasy or amphetamine.

However, because these fell into different categories of control under British law -- with drugs such as cocaine and heroin falling into the most serious Class A category -- no prosecution was possible.

Likewise no action was taken against Winehouse earlier this year after she was shown in a video published by a national newspaper apparently smoking what the paper said was crack cocaine.

"I was quite clear that I expressed my concern over the Kate Moss story, and we did that same investigation and we hit the same issue," Blair said.

"And I think it's reasonable for a jury to be able to say, beyond reasonable doubt, 'I can see that behaviour, you convince me that you're taking talcum powder because ... that's an unusual way to take it'."

However, Blair's call received short shrift from the CPS who said it appeared the country's top officer had "completely misunderstood the law".

"The issue was not whether the white powder that Kate Moss was snorting was cocaine or talcum powder. The law required us to prove that it was either a Class A drug or a Class B drug," said Director of Public Prosecutions, Sir Ken Macdonald.

"Any suggestion that the CPS does not prosecute celebrities is completely untrue -- we will prosecute when the police provide us with sufficient evidence to do so."