Cannabis growers are routinely being let off by police, the Mail can reveal today.

Even those cultivating scores of plants, potentially worth tens of thousands of pounds, are escaping with cautions.

One police force discovered 194 cannabis farms over four years but brought charges against only 79 of the suspects. 

The rest were cautioned or given warnings by Devon and Cornwall Constabulary.

Officers in West Yorkshire, Suffolk, Essex and Kent uncovered cannabis factories that had specialist heating and lighting. But again no charges were brought, even where stolen property and a firearm were found.

One grower of 150 cannabis plants in Sunderland – potentially enough to make 45,000 so-called spliffs – escaped sanction.

Campaigners said it was wrong to decriminalise a drug linked to mental illness, organised crime, violence and road deaths.

'People growing cannabis for commercial purposes should not get away scot-free,' said David Raynes of the National Drug Prevention Alliance.

'There is an argument that those who grow one or two plants for personal use could be cautioned, depending on their personal history. 

Police forces that do not charge people who have been caught with commercial quantities of cannabis are making a serious mistake.

'Of course they have discretion to deal with crimes as they see fit. But this creates a culture in which cannabis is 'nothing to worry about'. But it should be.'

Mr Raynes said by failing to prosecute police were 'failing society and building up problems for the future'.

Cannabis plants vary in size but in ideal conditions could yield up to 4-5oz of the drug, easily enough for more than 300 joints. Many of those given cautions are growing the drug to make easy cash through supplying friends, neighbours and colleagues. Official figures show that between 2011 and 2014 the number taken to court for growing cannabis fell by 87 per cent.

Police seized just under 400,000 plants last year, almost half the total from 2009-10. The number of cannabis farm raids has fallen dramatically, from around 16,500 in 2011-12 to under 10,000 last year.

The number sentenced for producing class B drugs such as cannabis fell by almost a third between 2013 and 2015.

Britain is a cannabis hot-spot, with almost £1billion worth of the drug changing hands each year, often in its most potent skunk form.

A 'problem profile' published by chief constables highlights how its production is linked to illegal immigration, people smuggling and organised crime.

Sara Thornton, who leads the nation's police chiefs, has admitted officers had given up investigating small scale cannabis farms, saying it had 'never been a top priority'.