The union balloted all members of the 44-strong chorus because of plans by ENO management to cut at least four jobs and impose a 25 per cent pay cut, Equity said.
The ENO has said it will cut the contracted time for members of the chorus by 25 per cent by moving them to nine-month contracts and has put forward a 17.5 per cent pay reduction, with the new contracts taking effect on 1 August 2017.
Forty-three chorus members responded to the strike ballot, with all of them supporting strike action and industrial action short of a strike.
Equity said in a statement that strike action would take place during the first act of the opera Akhnaten on 18 March.
Days of action short of a strike will take place between 4 March and 19 March, which Equity said might include protests during performances or rehearsal and costume calls. Equity said the chorus might not agree to work overtime.
ENO lost £5m of its grant from ACE last year, leaving it at £12.4m. The Arts Council said it was doing this because it had concerns about the charity’s governance and business model.
Hilary Hadley, the head of live performance at Equity, said: "The chorus have no interest in causing economic harm to their own company, which is why the overwhelming majority of the industrial action will be actions of protest and will not damage a performance in any way for the audience or the ENO box office. However, the impact of the current proposals on the chorus is so great that they have had to reluctantly consider strike action."
In a statement, a spokesman for ENO said the company was extremely disappointed by the decision to take industrial action "before undertaking serious negotiations".
The statement said: "Given the assurance that no action would be taken which would damage the company, we are saddened that the action that they have chosen seems specifically designed to cause the utmost harm to both the company and our audience.
"We are confident that we can reach an agreement with Equity and that we can work together to find a solution which both enables us to face the reality of a £5m cut in core funding while protecting the artistic integrity of our work."
ENO’s statement said that chorus members would also have first refusal on freelance work outside their contracts at a 100 per cent freelance fee.