Equity said its members will strike on September 21 following insufficient progress during talks with management on Friday.
The union said the walkout will hit the performance of Rigoletto.
Equity is calling for urgent talks with management to find a workable alternative with the hope that engagement early next week can avert the strike.
Simon Curtis, Equity national official for Wales, said: “Today’s meeting with WNO management made some progress but was ultimately disappointing. There was little engagement with the real concerns of chorus members around livelihoods and an unacceptable level of job losses.
“We have pushed for revised proposals and remain open for talks and want to engage constructively to find a way forwards which safeguards jobs and pay within the chorus.
“These professional singers deserve a greater degree of respect than has been shown to date. We ask again for management to balance their budget in a way which safeguards the world-class chorus which WNO rightly describes as part of its artistic backbone.”
Equity said WNO management is still proposing to cut the size and salaries of the chorus, and the threat of compulsory redundancy remains on the table.
Equity members previously voted 93% in favour of taking strike action on a 100% turnout.
