The smoking ban in prison has led to a rise in inmates using e-cigarettes to take hallucinogenic drugs, prison guards have said.

And it has led to wardens in jails unwittingly getting high.

Guards at five of Scotland’s 14 prisons were quizzed in a Stirling University study. They welcomed the ciggies ban behind bars but added that e-cigarettes were an “effective way” for cons to take drugs.

Vaping by prisoners is widespread, with more than 50 per cent using nicotine replacement therapy devices.