The DUP's Gregory Campbell and Willie McCrea have joined him in pressing Home Secretary John Reid to acknowledge the modern, super-strength version of the drug can be "deeply damaging".

The Ulster trio are among 44 MPs who have signed a motion by Tory MP Humfry Malins urging the Government to be aware of the dangers of cannabis and rethink its controversial 2004 decision to downgrade it to a 'class C' drug.

"The majority of class A drug addicts began by experimenting with cannabis," Mr Malins claimed.

In January 2004, then-Home Secretary David Blunkett reclassified cannabis from class B to class C. Two years later his successor, Charles Clarke, resisted pressure to reverse the decision despite disturbing evidence. Mr Clarke accepted the drug could trigger serious mental illness but pledged a publicity campaign to warn of the dangers.

According to the Home Office, since the reclassification most offences of cannabis possession by adults result in a police warning on the spot and confiscation of the drug.

There is a presumption against arrest, except where public order is at risk, where children are vulnerable or for repeat offenders. Young people under 18 are given a formal reprimand or warning at a police station.