A total of 7,865 cannabis farms were found across the UK in 2011/2012. Reuters

The former head of MI5 Baroness Mannigham-Buller has joined a group of MPs to call on the government to consider decriminalising cannabis.

Baroness Eliza Manningham-Buller believes that the current "war on drugs" is not working and has called believes a commission should be set up to examine the controversial move of relaxing the current laws.

In a speech at a meeting held by the All Party Parliamentary Group on Drug Policy Reform on Thursday, Baroness Manningham-Buller said the "war on drugs" has proven "fruitless" and that drug policy should no longer be a "taboo subject."

"Given its effects on us all and the enormous cost of our efforts to limit the problems which drugs cause, I find it extraordinary that it is not at the forefront of national debate.

"Instead there is a presumption that the current policy is the best we can do. And there is knee-jerk opposition to any change."

Baroness Manningham-Buller also said at the meeting that Britain should follow countries like the Czech Republic and Portugal, where possession and use of small quantities of drugs has been decriminalised.

"I think we need to look to the evidence. We have evidence from a lot of different countries about what works rather better than what we do in the UK," she said.

"The Czech Republic and Portugal have decriminalised possession and use of small quantities of drugs. They have lower levels of problem drug use, lower levels of use of these drugs among young people, lower cocaine use, lower heroin use.

"It's fairly clear that you do quite well if you have decriminalisation, so that's one of the policies we think needs to be looked at."

"Should we follow Portugal's fascinating example and focus on drug use as a health issue rather than a crime issue?"

The suggestion has come in criticism by policy director at the Centre for Social Justice Christian Guy, who sees decriminalising as "waving the white flag."

"I would agree that the war on drugs has been failing but what I'm concerned about is the idea we should stop fighting it and wave the white flag to these criminal gangs and say we are going to give up.

"We need to look at preventing drug use and deal with demand, we need to look at supply, we also need to look at treatment. We need to become more effective at getting people off drugs and we think this is a distraction."

Three years ago, the then Labour government reclassified cannabis from a Class B to a Class C.