The miners' strike of 1984–85 was a major industrial action to shut down the British coal industry in an attempt to prevent colliery closures. It was led by Arthur Scargill of the National Union of Mineworkers (NUM) against the National Coal Board, a government agency. Opposition to the strike was led by the British government of Prime Minister Margaret Thatcher, who called Scargill "the enemy within." The NUM was divided over the action and many mineworkers, especially in the English Midlands, worked through the dispute. Few major unions supported the NUM, primarily because of the absence of a vote at national level. Violent confrontations between flying pickets and police characterised the year-long strike, which ended in a decisive victory for the government and led to the closure of most of Britain's collieries. It was "the most bitter industrial dispute in British history." At its height, the strike involved 142,000 mineworkers. The number of days of work lost to strike was over 26 million, making it the largest since the 1926 General Strike.
The NCB was encouraged to gear itself toward reduced subsidies in the early 1980s. After a strike was narrowly averted in February 1981, pit closures and pay restraint led to unofficial strikes. The main strike started on 6 March 1984 with a walkout at Cortonwood Colliery, which led to the NUM's Yorkshire area's call to make the strike official. The union strategy was to cause a severe energy shortage of the sort that had won victory in the 1972 strike. The government strategy, designed by Thatcher, was threefold: to build up ample coal stocks, to keep as many miners at work as possible, and to use police to break up attacks by pickets on working miners. The critical element was the NUM's failure to hold a strike ballot, which enabled a minority to keep working and kept other unions from supporting it.
The strike was ruled illegal in September 1984, as no national ballot had been held. It ended on 3 March 1985. It was a defining moment in British industrial relations, the NUM's defeat significantly weakened the trade union movement and was a major victory for Thatcher and the Conservative Party and the Thatcher government was able to consolidate its economically liberal programme. The number of strikes fell sharply in 1985 as a result of the "demonstration effect" and trade-union power in general became diminished. Three deaths resulted from events related to the strike.