Cannabis is now legal for recreational use in 17 states and in Washington, D.C., and has been decriminalised in a further 13 states.

However, police forces have struggled to retrain their sniffer dogs to indicate on other illegal drugs but not on cannabis, The Times reports.

Don Slavik of the US Police Canine Association said: “Once you train a behaviour in a dog, that never goes away.”

Bedford County Sheriff Mike Miller added: “We won’t use our dogs trained in marijuana because that could be a defence an attorney would raise to say, ‘Which odour did the K9 alert on, was it marijuana or an illegal drug?”