The amount of staff that have tested positive to use of Class A drugs – such as heroin, cannabis, cocaine, amphetamines and methadone – increased by 50 per cent between 2007 and 2011 from 2.26 per cent to 3.23 per cent, according to statistics from the drug and alcohol screening company Concateno.

Although the figures did not split out the public sector from the private sector, healthcare workers and police staff were among the 1m employees who had been found to have taken the drugs. The other sectors were logistics, haulage, utilities, retail, occupational health, manufacturing, construction and commerce.

The research also revealed that most users of Class A drugs were between 25 and 34 years old.

Dr Claire George, Concateno’s laboratory director, said: “These are conservative figures across the workplace.”