The Communist Party of Great Britain (CPGB) was the largest communist organisation in Britain. It was established on 31 July 1920 and dissolved on 23 November 1991. For most of its existence, it aligned itself with the USSR and received funding from it. The British surveillance state saw it as the main subversive threat to the United Kingdom for much of the 20th century, and was consequently the target of many operations by MI5.
The CPGB was one of the few groups on the left in Britian that the SDS did not target. It is often remarked that the CPGB did not present any issues regarding public order. While there is the occasional mention of it in reporting, the majority of disclosure regarding the CPGB instead relates to the Whitehall 'subversion' committees.