The revelation that CR might be used fired from a water cannon to disperse crowds in the UK has disturbed civil rights campaigners.
The chemical is more volatile than CS and it causes sore eyes and breathing difficulties. Although its effects are not thought to be lasting, it causes extreme discomfort.
Officials disclosed that they had CR in their stocks as part of Britain's declaration under the Chemical Weapons Convention, which has not been published despite an earlier pledge to do so.
They would not say, though, how long they had held the chemical, where it was kept or in what circumstances it would be used. Nor would they say whether the government also had stores of water cannon from which to fire it into crowds.
The police have said they do not keep CR, so it seems it would only be used in circumstances where the army was called in to support them against rioters.
CR, the full name of which is Dibenz-1:4-Oxazepine, has not been used in the British Isles before although scientists at the Porton Down defence research centre in Wiltshire are known to have experimented with it about 20 years ago.
Harry Cohen, the Labour MP for Leyton and Wanstead, plans to table a series of parliamentary questions to find out more about the stockpile.
"Why is it that the Ministry of Defence has a greater variety of riot control agents than the civil police? I shall be demanding some answers," he said.
The civil rights group Liberty is also angered by the disclosure. It already campaigns against CS, saying there has been no consultation or parliamentary scrutiny of its use.
Guidelines laid down by the Association of Chief Police Officers suggest CS should only be used by individual officers under attack, the group says.
John Wadham, Liberty's director, said neither CS nor CR should be used to control crowds. "We see it as part of a move towards policing by coercion rather than by consent," he said.
It does not seem that the stocks are destined for Northern Ireland. The Royal Ulster Constabulary, under whose umbrella the army acts in the province, said yesterday that it preferred plastic bullets.
"They are a much preferred means of riot control in any circumstances. Water cannons and CS gas are indiscriminate while plastic baton rounds can be directed at a particular rioter," a spokesman said.
Officials from the Department of Trade and Industry and Ministry of Defence would not say how much CS was held, where it was held or what plans there were for using it.
Although the Conservatives promised to publish the Government's submission under the Chemical Weapons Convention, which includes riot control agents, it has only released the parts relating to old stocks and ongoing defensive research.
In a separate development, it has also emerged that the Ministry of Defence has let a pounds 100,000 contract to scientists at Porton Down to research alternatives to CS gas.
The sprays are mainly used by police forces to counteract attacks on individual officers.
Figures released by ACPO after a trial period reported a 6.8 per cent drop in assaults against officers armed with the spray, but a 14 per cent drop in assaults against a control group.Reuse content