The union said it would ballot 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.
Cressida Pollock, chief executive of the ENO, said in a statement that because of the grant cut the charity was remodelling its business, which meant that chorus, orchestra and back-stage staff contracts would have to be reviewed.
She said the financial situation at the ENO also necessitated changes in the way shows were produced and where they were performed, and the charity needed new strategies to raise money and increase box-office revenues.
The ENO has said it will reduce its main stage season at the London Coliseum from 10 to six performances a year until 2020 and has cut the size of administration teams by 30 per cent over the past decade.
Equity’s statement said the management of ENO was "in freefall" and warned the charity was at risk of "terminal decline".
Hilary Hadley, head of live performance at Equity, said: "The plans by ENO management amount to cultural vandalism – they are threatening the future of a world-class chorus and, as a consequence, the viability of ENO itself. We have no alternative than to ballot on industrial action, which could result in strikes and cancelled performances."
Pollock’s statement said: "It is not our intention to further casualise the workforce at ENO. We remain committed to retaining a permanent chorus, orchestra and back-stage team.
"While we remain committed to our permanent ensembles, we also recognise that we have to make significant cost savings in order for ENO to be a more sustainable organisation in the long term. We are working towards a solution with the ENO chorus that would see it maintained as a permanent ensemble but with a greater degree of flexibility in terms of contracts, which would reflect the economic reality of ENO’s situation.
"We are looking at how we can reduce our reliance on freelance back-stage staff in order to preserve our back-stage ‘family’ and protect full-time positions – these people provide the backbone to our company."
According to the Charity Commission website, the ENO had an income of £38.2m in the year to 31 March 2015, and spent £38.1m.