Billy Caldwell, who uses cannabis oil to control his severe epilepsy, was the subject of a public outcry over the oil’s use

The public now overwhelmingly backs the full legalisation of cannabis, according to a poll that suggests there has been a seismic shift in opinion since the government sanctioned medical use of the drug.

Almost two in three people say that they support legalisation, the Populus poll found. This is a radical turnaround from the last poll in May, when support and opposition were almost even, at 43 per cent and 41 per cent respectively.

In the intervening period the government has approved the medicinal use of cannabis after a public outcry over the case of Billy Caldwell. The 12-year-old boy was admitted to hospital in a life-threatening condition after the Home Office seized the cannabis oil that was helping to control his severe epilepsy.