Researchers found persistent teen users of the drug, who started smoking it at school, had lower IQ scores as adults.

They were also significantly more likely to have attention and memory problems in later life.

Academics at King’s College London and Duke University in the U.S., those who  used it heavily as teenagers, but quit as adults, did not regain their full mental powers.

The study revealed about one in 20 admitted to starting cannabis use before the age of 18, while a further one in 10 took up the habit in the early or mid 20s.

The cognitive abilities of the 10% of people who started smoking in their 20s, who could loosely be classed as college smokers, also suffered while they were still smoking.

However, if they gave up at least a year before their IQ test at 38, their intelligence recovered, suggesting their brains were more resilient than teenage brains and bounced back.