During the Donoughmore period of political experimentation, several leftist parties were formed. Unlike most other Sri Lankan parties, these leftist parties were noncommunal in membership. Working-class activism, especially trade unionism, became an important political factor during the sustaiÍÍÍÍned economic slump between the world wars. The first important leftist party was the Labour Party, founded in 1931 by A.E. Goonesimha. Three Marxistoriented parties--the Ceylon Equal Society Party (Lanka Sama Samaja Party--LSSP), the Bolshevik-Leninist Party, and the Communist Party of Sri Lanka (CPSL)--represented the far left. All three were divided on both ideological and personal grounds. The Soviet Union's expulsion of Leon Trotsky from the Communist Party after Lenin's death in 1924 and Stalin's subsequent decision to enter World War II on the Allied side exacerbated these differences, dividing the Communists into Trotskyites and Stalinists. The LSSP, formed in 1935 and the oldest of the Sri Lankan Marxist parties, took a stance independent of the Soviet Union, becoming affiliated with the Trotskyite Fourth International, which was a rival of the Comintern. Most LSSP leaders were arrested during World War II for their opposition to what they considered to be an "imperial war." Although in more recent years, the LSSP has been considered a politically spent force, gaining, for example less than 1 percent of the vote in the 1982 presidential elections, it has nevertheless been touted as the world's only successful Trotskyite party.

The CPSL, which began as a Stalinist faction of the LSSP that was later expelled, formed its own party in 1943, remaining faithful to the dictates of the Communist Party of the Soviet Union. The Bolshevik-Leninist Party was formed in 1945 as another breakaway group of the LSSP. The leftist parties represented the numerically small urban working class. Partly because these parties operated through the medium of trade unionism, they lacked the wider mass appeal needed at the national level to provide an effective extraparliamentary challenge to the central government. Nonetheless, because the leftists occasionally formed temporary political coalitions before national elections, they posed more than just a mere "parliamentary nuisance factor."

Data as of October 1988