Smoking cannabis as teenager could increase the risk of depression in adulthood by almost 40 per cent, a landmark study by Oxford University has found. Researchers warned that use of the drug in adolescence may be responsible for around 60,000 cases of the condition in subsequent decades - around one in 14 of all cases.

They urged parents to be less relaxed about teenage use of cannabis, warning that some of the strains being pushed today are 10 times as strong as the drugs which were dealt 30 years ago.

And they called on ministers to prioritise prevention of drug taking, warning that the damage caused by cannabis was now a public health risk with “devastating” consequences.

The research examined 11 major studies involving more than 23,000 individuals from adolescence until their 30s.

It found that those who tried cannabis in their teens had a 37 per cent higher risk of depression between the ages of 18 and 32, and more than three times the risk of attempting suicide.