The National Socialist Party was a small political party in the United Kingdom, founded in 1916. It originated as a minority group within the British Socialist Party who supported British participation in World War I; while historically linked with the Marxist left, the party grew more moderate. It affiliated to the Labour Party and was eventually absorbed by it.
The National Socialist Party was founded by Henry M. Hyndman and his followers after his defeat in the leadership elections of the British Socialist Party. They believed that it was desirable to support the United Kingdom in World War I against "Prussianmilitarism". Although maintaining that they were a Marxist party, after affiliation to the Labour Party in 1918, they renounced vanguardism and saw in the Russian Revolution only the danger that it might weaken the United Kingdom's war effort. The party was grouped around the newspaper Justice.
In 1919, the group changed its name to the Social Democratic Federation, reverting to the name that the British Socialist Party had used. At one point eleven MPs were members, but after the death of Hyndman in 1921, the group gradually dissolved into the Labour Party. The party sponsored several candidates at each election until 1924, all of whom ran for Labour. After 1924, its MPs were instead sponsored by their local Labour Party. The party finally disbanded in 1939 due to a lack of funds, although some remaining members formed a "Social Democratic Fellowship".