CS spray is being used as an offensive weapon by the police in almost four out of 10 cases, despite guidelines that it should be used primarily in self-defence.
The Police Complaints Authority (PCA), which carried out research into the use of the spray, has also expressed concerns about CS causing burns to the eyes and face and being used against mentally ill people.
A survey of police forces in England and Wales during the year ending October 1999 discovered that the spray failed to incapacitate the target in nearly one-fifth of all cases.
The report, CS Spray: Protecting the Public?, published today, examined 135 complaints against officers using the hand-held device. CS is fired an estimated 3,000 times a year since being introduced to all but three of the 43 police forces in 1998.
The reports says that in 38 per cent of the cases examined, the primary use for firing the spray was not self-defence. "In some cases the subject of the spray was certainly not posing a threat and in one or two cases was actually running away," it says.
"The 1996 guidance stresses that the use of the spray is 'primarily for self-defence'... CS spray was 'sold' to the public and to the police as a method of self-defence, albeit as a last resort."
The study coincides with the announcement today that an officer of North Wales police should be charged with abuse of authority after he was accused of using the spray unnecessarily and recklessly against a member of the public during a fight outside a fast-food takeaway in Rhyl.
The report urges police to avoid using CS on the mentally ill, a practice that has been strongly criticised by mental health charities as extremely dangerous. Six of the 135 complainants had a mental illness. The PCA report says: "CS may exacerbate the side-effects of anti-psychotic drugs - breathing problems, changes in heart rate or allergic rashes."
Surprisingly, the CS has been found to be ineffective in one in every five uses. This can often be due to the influence of alcohol and drugs, which can mask its effects.
In eight of the complaints, serious but temporary injury was caused from CS. The most common injuries were burns to the face and eyes. The burns were caused by the solvent, MIBK, used in the device, rather than the CS chemical itself.
The PCA sayscaution should be used when using the spray against a person with a firearm because it "might cause that person to fire indiscriminately".
CS spray is credited in contributing to a 19 per cent fall in the number of assaults on officers since 1996. The three forces which have not introduced CS because of concerns about public safety - Nottinghamshire, Northamptonshire and Sussex - have had assaults on officers rise by more than 10 per cent.
The PCA report concludes that "CS spray does not appear to present a serious risk to the public". Among the recommendations are for forces to review training and guidelines, paying particular attention to spraying it at close range and against the mentally ill.
The Home Office minister Charles Clarke said: "Used within strict guidelines it [CS] will continue to be one of the tools available to police officers to ensure their safety and that of the public."