More than a quarter of police and crime commissioners have written to the policing minister calling for cannabis to be upgraded to a class A substance, The Mail on Sunday can reveal.

In the stark letter to Dame Diana Johnson MP, seen exclusively by this newspaper, 14 police chiefs claim the effect of the drug in society ‘may be far worse’ than heroin.

They warn that ‘we cannot allow this to become the Britain of the future’. And they also hit out at the recent report by the London Commission – backed by Labour London mayor Sir Sadiq Khan – which suggested decriminalising small amounts of cannabis, which is currently a class B drug.

‘Heroin can kill quickly but the cumulative effect of cannabis in our society may be far worse,’ the letter states. 

It adds that class A status – which comes with potential life sentences for suppliers – was the way forward ‘rather than effective decriminalising’.

And renowned psychiatrist Professor Sir Robin Murray, of King’s College London, told The Mail on Sunday that the UK may now be ‘at the beginnings of an epidemic of cannabis-induced psychosis’ which could overwhelm NHS mental health services.

The commissioners also pointed to other countries where laws are laxer, warning that the US has seen ‘unofficial pharmacies’ selling cannabis and the powerful opiate fentanyl alongside one another, while Portugal has been forced to consider reversing drug decriminalisation after a 30-fold increase in psychosis.

They said cannabis’s effects were so devastating it had ‘more birth defects associated with it than thalidomide’ – the notorious morning sickness drug which caused deformities among thousands of babies in the 1950s and 1960s.

Their warnings came after Marcus Monzo, 37, was last week found guilty of 14-year-old Daniel Anjorin’s murder after he attacked him with a samurai sword in Hainault, east London, while in a state of cannabis-induced psychosis.

David Sidwick, Police and Crime Commissioner for Dorset, said cannabis legislation was ‘clearly not fit for purpose’ and likened it to ‘using a machete for brain surgery’. 

He added the public wanted to see ‘tougher measures’ for cannabis possession because it was a gateway to harder drugs.

His Devon and Cornwall counterpart Alison Hernandez said: ‘The fact that we’ve been so blase about cannabis in society means that people think it’s legal and normal, and it’s not. 

‘We’ve got to show them that it’s not, and the way you do that is to be quite fierce in your enforcement arrangements.’

Latest figures show three in four people caught with cannabis avoid appearing in court, while 87 per cent of children and young people in alcohol and drug treatment cited cannabis dependency, compared to 39 per cent for alcohol.

Stuart Reece, an Australian clinician and cannabis researcher quoted in the letter said more than 90 per cent of hard drug addicts he encountered had started with cannabis.

He said pro-cannabis campaigners had the view it was ‘my right to use drugs and destroy my body and you will pay for it through the NHS’.

Dr Karen Randall, a physician in the US state of Colorado where recreational cannabis was legalised in 2012, said healthcare costs linked to the drug are ‘exorbitant’.

A Home Office spokesman said: ‘We work with partners across health, policing and wider public services to drive down drug use, ensure more people receive timely treatment and support, and make our streets and communities safer.’