The £5 deals are known as Mach 5s and the crack cocaine sprinkled on them makes them more addictive and dangerous to users.

Police believe a number of people who buy cannabis are unaware of crack cocaine being mixed into amounts of the class C drug.

Crack cocaine is an addictive class A drug and police are asking people to be on the alert and be aware of the symptoms associated with early crack cocaine use.

These can include extremes of behaviour often marked by increasing anxiety and paranoia. Users may also experience a high temperature and loss of appetite.

Insp Andrew Briggs said: "We have become aware that dealers are trying to introduce cannabis users to harder drugs by lacing small amounts of cannabis with crack cocaine.

"Using any drugs obviously poses a risk to health, but the practice of introducing a highly addictive class A drug to people under the pretext of dealing a small amount of cannabis is an extremely dangerous precedent.

"There is no quality control when using illegal drugs and the bottom line is that people are never truly aware of what is contained in any drugs they are taking.

"We are following a number of leads regarding the so-called Mach 5s, but I would appeal to anyone who believes they may have been affected to contact Crimestoppers anonymously on 0800 555 111."