The unpublished study, completed a year ago by Porton Down, an organisation that evaluates Britain's defence against chemical and biological weapons, suggests the solvent is too dangerous to use.
The Home Office and Department of Health, however, argue that people are sprayed with such a small quantity of chemicals that there is no long- term harm.
But the report, Literature Review of Solvents Suitable for the Police, is certain to add to concern about the use of the spray, which has prompted scores of complaints and has been linked to several deaths. Three police forces are still refusing to use the weapon,which has been issued since 1996.
The latest controversy centres on the use of the solvent, methylisobutylketone (Mlbk), which is used to dissolve CS powder so it can be sprayed in solution into a person's face. Researchers tested 16 possible substances, including Mlbk, and graded them from nought to four in order of toxicity. Mlbk was graded three and "serious" toxicity, while only two of the chemicals were considered safe enough to use. Further tests, however, found that these were not suitable for dissolving CS powder.
An alternative chemical that is currently banned in Europe, but may soon be given the all- clear, could provide a safer alternative.
The current study followed an earlier report, also by Porton Down, in 1996, which also warned about Mlbk.
The Department of Health ruled during police trials that the combination of Mlbk and CS powder was "suitable" because there was no evidence of any long-term harm. A Home Office spokesman stressed: "We are continuing to review possible alternatives."
The Police Complaints Authority has received more than 250 complaints about CS spray and is investigating its use in the deaths of two men.