A wildcat strike action, often referred to as a wildcat strike, is a strike action undertaken by unionized workers without union leadership's authorization, support, or approval; this is sometimes termed an unofficial industrial action.
In 1965, Canadian postal workers illegally walked out for two weeks and won the right to collective bargaining for all public sector employees. This resulted in them throwing out the leadership of the company union and forming the Canadian Union of Postal Workers.
On March 23, 2012, Air Canada ground employees suddenly walked off the job at Toronto Pearson International Airport, resulting in many flight delays, after three workers were suspended for heckling Canadian Labour Minister Lisa Raitt. This followed months of fighting between Air Canada and its other unions.
Wildcat strikes were the key pressure tactic used during the May 1968 protests in France.
Wildcat strikes have been considered illegal in the United States since 1935. The 1932 Norris-La Guardia Act provided that clauses in labor contracts barring employees from joining unions were not enforceable, thus granting employees the right to unionize regardless of their workplace situation. Unions have the power to bargain collectively on behalf of their members and to call for strikes demanding concessions from employers. Under the 1935 National Labor Relations Act (NLRA), federal courts have held that wildcat strikes are illegal and that employers may fire workers participating in them.
Nevertheless, US workers can formally request that the National Labor Relations Board end their association with their labor union, if they feel that the union is not adequately representing their interests. At this point, any strike action taken by the workers may be termed a wildcat strike, but there is no illegality involved, as there is no longer a conflict between sections 7 and 9(a) of the NLRA.
Some strikes that begin as wildcat actions, such as the Memphis Sanitation Strike and Baltimore municipal strike of 1974, are later supported by their respective unions' leadership (who then begin fulfilling their obligation to collectively bargain for their worker-members).
In 2018, West Virginia teachers went on strike to demand higher wages and, crucially, comprehensive and affordable health coverage. Without the sustained sanction of union leadership, this strike became a wildcat strike. In 2018, wildcat strikes by teachers demanding better pay and school funding have also occurred in Oklahoma, Kentucky, and Arizona.
In Vietnam, all workers are required to join a union connected to the Vietnam General Confederation of Labor. Due to workers' distrust of this agency, nearly all strikes in the country are wildcat strikes.
Although the 2018 West Virginia teachers' strike is sometimes referred to as a wildcat strike, labor unions called for a vote on authorization to take action in late January 2018. The vote passed. Labor leaders formally announced a statewide walkout beginning February 22, 2018. During the ensuing nine days of the walkout, all county superintendents throughout the state canceled school. 
There are some cases where union recognition of a strike is complicated. For example, during the year-long British miners' strike of 1984-5, the national executive supported the strike but many area councils regarded the strike as unofficial, as most ballots at area level had produced majority votes against the strike and no ballot was ever taken at national level.