Hundreds of Rolls-Royce (RR) workers and Amicus members are attending a rally in Derby in support of a sacked union convenor.
Workers rally in support of Amicus union convenor Jerry Hicks
The rally is being held at the Moorways Sports Stadium as the union said it was not allowed to use RR land.
Sacked convenor Jerry Hicks, who worked at the RR Bristol plant, will speak.
Workers attending will have their pay stopped for the duration of the rally. The union has suggested strike action could later hit the Derby plant.
Amicus' regional secretary Nev Jackson said: "Trade unionists within Rolls Royce and outside are appalled by the treatment of Jerry and are supporting the strike and campaign for his reinstatement.
"The Derby rally, which people will attend in their own time, will demonstrate to the company our solidarity and our intention to stand up and fight for trade union rights."
Nearly 100 test engineers from the Bristol plant began continuous strike action on Tuesday in protest at his dismissal in July. Amicus said it cannot rule out a spread of industrial action.
Mr Hicks was dismissed after allegations of misconduct but an employment tribunal provisionally found he had "probably been dismissed on trade union grounds".
The company said Mr Hicks had been sacked for organising unlawful industrial action and attempting to undermine disciplinary procedures - claims he denied. His case is due before a full hearing within six months.