Industrial Action is a concerted stoppage of work, or action short of strike that is taken by employees in contemplation or furtherance of a Trade Dispute. Official Industrial Action is action which is called by someone who is authorised to do so by the Trade Union’s Rule Book. Unofficial Industrial Action is action which is called by someone who is not authorised to do so and is viewed as unlawful. In order to preserve immunity from legal action by an employer a Trade Union must take steps to repudiate unofficial industrial action. Lawful industrial action is action which has been carried out in response to a ballot for Industrial Action.
DfE Code of Practice - Industrial Action Ballots and notice to Employers