Cannabis resin has also dropped in strength from 3.4% to 2.6% over the same period.

These figures contradict the arguments given by the UK government for reclassifying cannabis from Class C to Class B.

In May, Home Secretary Jacqui Smith told parliament that the strength of cannabis had increased 300% and there was a 'causal link, albeit a weak one, between cannabis use and psychotic illness.'

"My decision takes into account issues such as public perception and the needs and consequences for policing priorities," she said.

"There is a compelling case for us to act now rather than risk the future health of young people."

Official figures also show a drop in cannabis usage since the drug was downgraded to Class C.

"This information suggests that, in the time that it has been a class C drug, usage levels of cannabis have fallen and so has its strength," said David Porteous, a lecturer in criminology at Middlesex University.

"These findings make a mockery of the decision to re-reclassify cannabis and of the government's wider claim to base policy-making decisions on scientific research."

"Furthermore they call into question the validity of other controversial and publicly criticised government claims regarding drug policy, for example the link between cannabis and mental illness or the legitimacy of our current classification system."

But when the FSS were asked to comment on the figures, they stated that the sample was too small to be statisically significant.

A spokesperson claimed that the figures were 'unlikely to be an accurate representation of THC in cannabis across the board as not all samples submitted to the FSS are routinely analysed for THC content.'

"The FSS database also does not distinguish between sinsemilla cannabis and imported herbal cannabis," said the spokesperson.