Union membership had grown to over four million by AD 1913. The early years of the 20th century were full of industrial unrest caused by a fall in living standards. During World War I, the Treasury Agreement of 1915 between the government and the unions (without the miners) banned strikes, but allowed collective bargaining. Despite the Agreement there were unofficial strikes during the war, but it set a precedent for contact between government and union leaders.

Consensus broke down during the Depression of the 1920s and bitter class conflict led to the General Strike of 1926. Unions lost membership in the 1930s, but gained from the new mood of cooperation during World War II. The powerful leader of the Transport and General Workers Union, Ernest Bevin, was appointed Minister of Labour by Churchill in 1940. Under him, unions and government worked together to improve industrial conditions.

In 1945, the Trades Union Congress produced a list of priorities including nationalisation of key industries and public services and a welfare state. The collaboration with the post-war Labour Government (1945-51) was probably the high point of union fortunes. They fluctuated wildly during the rest of the century, reaching their lowest point under the Thatcher Governments (1979-90), which drastically reduced union power.