The Workers' International League was a Trotskyist political party in the United Kingdom. It was formed in 1937 by around ten members of the Marxist Group, who had split due to malicious rumours spread concerning the activity of Ralph Lee then a newly arrived South African comrade. The new group, led by Jock Haston and Ralph Lee also included Gerry Healy and Ted Grant.

The group remained in the Labour Party, where they distributed the magazines Youth for Socialism (soon renamed Socialist Appeal) and the Workers International News. The WIL grew with recruits from the Labour Party, the Communist Party of Great Britain, the Independent Labour Party and the Marxist Group.

The Fourth International was formed in 1938, and the WIL refused to merge into the newly formed official British affiliate, the Revolutionary Socialist League itself a regroupment of the Marxist group and others. They requested either affiliate or sympathiser status to the international but were rejected.

With the outbreak of World War II, the WIL expected to be banned and so temporarily moved a few members to Dublin. It soon became obvious that the group would not be persecuted, and they were allocated paper for their publications. Four members of the organisation attempted, without approval from the party, to purloin some cards to exempt certain members from military duty for medical reasons but this was discovered, and those involved were imprisoned.

Unlike the Revolutionary Socialist League, the WIL readily adopted the Proletarian or American Military Policy developed by Trotsky in his last writings and advocated by the Socialist Workers Party. They campaigned for deep air raid shelters for workers, and after 1941 against the pro-war, anti-strike position of the CPGB.

The WIL began to orientate towards the trade unions, and deprioritised entrism into the Labour Party. They formed the Militant Workers Federation with ILP members and some anarchists, and encouraged and supported militant trade union activity. By 1943, they had forced the Royal Ordnance Factory in Nottingham to accede to workers' control.

By 1944, the Fourth International had realised that the WIL were far more effective and closer to the FI's policies than the RSL which had disintegrated into a set of warring factions, and so coordinated a unity conference. This produced the Revolutionary Communist Party, which adopted all the WIL's positions.