The use of informers and agents provocateurs by both the government and the police has not been uncommon in British history. Edward Thompson has made lengthy references to the employment of spies by the authorities in the period from 1790 to 1830. The highest levels of the London Corresponding Society were penetrated in 1794 by an informer known as “Citizen Groves”, and, following this, use was made of informers in combating the Luddite movement, and in the Pentridge Rising, the Despard and Spa Fields Affairs, and, most important of all, the Cato Street Conspiracy of 1820. The use of political spies is also known to have occurred during the First World War and immediately afterwards, and allegations were made in Parliament in this respect during the period of the General Strike in 1926. The recent opening of the files of the Metropolitan Police for the 1930's has revealed that informers were also used during the unemployed disturbances of these years, in particular in the attempt to prevent the outbreak of violence during the marches on London organised by the National Unemployed Workers' Movement (NUWM) in 1932, 1934 and 1936, and on the occasion of the National Joint Council of Labour demonstration in Hyde Park in February 1933, although in these instances it has not been possible to establish the identity of the person or persons concerned.