Staff at five Gala casinos in London are threatening to walk out because of poor working conditions and a sub-inflation pay offer.
After rejecting a rise of 2.25%, 90% of 500 GMB and T&G union members across the sites have voted in favour of industrial action.
"Members are fed up with the company ignoring them and not taking them seriously on pay and other important issues such as health and safety," said GMB spokesman Mike Ainsley.
In addition to the wages dispute, staff are unhappy at having to stand while inspecting gaming tables and the dangers of working in an environment with passive smoke.
"Gala won't do anything about smoking unless the Government tells them to. One person has already had to retire because he couldn't continue working in the smoky atmosphere," said Ainsley.
A Gala spokesman said: "We are confident that we can reach a satisfactory solution for all sides when we meet this week."