Cocaine use has almost trebled in seven years with the NHS warning of an epidemic in the numbers hooked on the class A drug.

Nearly one in four suspected drug users now tests positive for cocaine compared with only one in 10 when Labour came to power.

The figures, based on the first accurate audit of more than 80,000 blood samples analysed by the NHS, show the increasing size of the burden facing the health service as a result of cocaine use.

More than 22 per cent of samples are testing positive compared to around 9 per cent seven years ago. Until now, the size of the UK cocaine problem has been based on estimates extrapolated from drug seizures, convictions and self-reported use. The research, which appears in the journal Forensic Science International this week, is the first to use real data rather than estimates.

Police are particularly concerned about crack which is highly addictive and linked to a rise in violent crime.