Cannabis warnings go up by 50 per cent

Figures show about 30,000 were given official warnings, compared with just over 21,000 issued in 2005/06.

The Met said today the warnings are "useful and effective" and their rising number reflects efforts to tackle crimes such as street robbery and burglary.

A spokesman said: "They are a bi-product of tackling and disrupting priority crime as well as dealing with issues raised by communities who see cannabis use fuelling anti-social behaviour and other low-level crime.

"Cannabis warnings are also time efficient in dealing with this crime and allow officers to remain on patrol and visible to the public."

The warnings system was introduced when the Government downgraded cannabis from a Class B to a Class C drug three years ago.

The number of warnings issued in London now makes up more than a third of those handed out across the UK.

But police insiders admit some borough commanders are using the sanction to boost crime detection rates.

Earlier this year, Surrey Chief Constable Bob Quick admitted officers were targeting minor offenders, such as shoplifters or people caught with cannabis, rather than hardened criminals in order to hit government targets. The result was official league tables were misleading, he said.

Met Commissioner Sir Ian Blair recently told the London Assembly: "The Met has some understanding of the comments expressed by Mr Quick."

However, he pointed out that the Met's detection rate for serious assaults was running at 33 per cent, the third annual increase in a row.

But one senior officer said: "There is a serious question over how we increase the detection rate. By using cannabis warnings and other means, such as fixed penalty notices, we are essentially creating crime to solve crime.

"The result is that we are victimising a significant number of young people by targeting them for offences such as cannabis possession."

The figures show wide variations between London boroughs in the number of cannabis warnings issued.

The biggest total of 3,371 was in Westminster, reflecting the size of the area and the high number of anti-terrorist stop and searches carried out there.

Other inner-city boroughs such as Brent, which recorded the second highest number of warnings - 1,978 - and Lambeth, Southwark, Camden and Hackney all issued higher-than-average numbers. The lowest number of warnings - 190 - was recorded in Sutton.