Dr Jackson said the authorities did not dare raid places such as university campuses because of the backlash.

A reaction from the rich and powerful parents whose children could be caught using drugs.

Yet poorer neighbourhoods were targeted by police, he said, while giving evidence to the Commons home affairs select committee on knife and gun crime and drugs.

- you go up on a couple of high-profile universities in Britain and you do a drug raid you will get a huge reaction from people of power whose children are going to be damaged by that process.

"Or you can let the same laws apply to those on the ghetto corners, have the same relaxed notion. At least give the others the same playing field."

He hit out at the tough anti-cannabis policy in the US.

"In America, we have made such a crime of marijuana for example; we have taken low-level drugs and made them as if they were assault weapons.

"It's not working. It's very costly."

The UK Government reclassified cannabis from Class C to Class B earlier this year, in defiance of its own independent advisers who recommended it be kept in the less serious category.

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