The Socialist Party of Great Britain, also known as the SPGB, is a small Marxist party, which is emphatically not Leninist. It should not be confused with the larger Socialist Party of England and Wales, a Trotskyist party. The SPGB experienced a split in 1991 into two groups, each named the Socialist Party of Great Britain.

The party is one of the oldest political parties in the UK, founded in 1904 as a split from the Social Democratic Federation. It was condemned by Lenin for refusing to join the Third International. It consistently argues against both vanguardism and any sort of reform, and refuses to engage in direct action.

The SPGB publishes a monthly paper, the Socialist Standard.

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