A strike is defined in Trade Union and Labour Relations (Consolidation) Act 1992 as "any concerted stoppage of work" (TULRCA 1992, s.246). This can include an overtime ban or a rest-day working ban called by a trade union (see Connex SE v RMT Union CA 1999 IRLR 249, CA). For the purposes of the rules on ballots authorising industrial action an overtime or call-out ban counts as "industrial action short of a strike" (Employment Relations Act 1999 sch.3 para 6(2)).

An employee taking part in unofficial industrial action at the time he is dismissed is normally disbarred from making any unfair dismissal claim (TULRCA 1992, s.237). He will be able to make such a claim only if he was dismissed for one of the "automatically unfair" reasons specified in TULRCA 1992, s.237(1A) such as dismissal in family, health & safety, working time, employee representative and whistleblowing cases.

Individuals who take part in official industrial action (technically "protected industrial action") have had increased protection against dismissal since 24th April 2000 ( TULRCA 1992 s.238A). The general effect is that the dismissal of an employee for taking lawfully organised, official industrial action is automatically unfair dismissal if:

- it takes place within twelve weeks (8 weeks until 6th April 2005) of the employee commencing industrial action; or
- it occurs after this 12 week period but the employee had returned to work before the end of the 12 weeks; or
- it occurs after this period and the employer has not taken all reasonable steps to resolve the dispute.

Subject to a defence of not knowing that workers were on strike, it is a criminal offence for an employment business to provide their staff to perform the duties of workers who are on official strike or who are taking other official industrial action.

There is no statutory definition, as such, of "official industrial action" or "official strike". However, there is a kind of definition, more of a description than a definition, of "unofficial strike" and "unofficial industrial action". Any strike (often called a "wildcat strike") or other industrial action which is not authorised or endorsed by a trade union will generally be "unofficial" (TULRCA 1992, s.237(2)). If a strike is endorsed by a trade union without that union having gone through the proper balloting procedure required for strike action pursuant to a trade dispute the union's funds will be at risk - it will not be entitled to claim immunity from civil action from those who have suffered loss (see Trade union matters/ballots and/or Trade union matters/immunity from civil action in certain cases ).

See also Trade union matters/industrial action and/or Strikes/a general note and/or Unfair dismissal/automatically unfair dismissals/industrial action .