Genting Casinos says 1,642 employees in the UK could face losing their jobs, despite plans to reopen venues.
The GMB union said there were also plans to close three clubs in Margate, Torquay and Bristol.
Genting said the coronavirus lockdown had forced it to make "heartbreaking decisions" about the future of its business.
Colleagues at risk remain on furlough and it hopes to bring them back "at a later stage", Genting said.
The firm said that it has faced "unprecedented challenges and heavy losses" over the past few months, as a result of the coronavirus pandemic.
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"Despite the ongoing uncertainty in relation to when we may be able to open our doors again, we are continuing to prepare for our re-opening - albeit with significant changes being implemented to our physical premises and to our operating model," said a Genting spokesman.
"In light of these changes, we are having to make some heart-breaking decisions about the future of the business and it is with huge regret that job losses are simply unavoidable."
Genting's plans to change its business model include ending poker games and hospitality services, according to GMB.
"This announcement by the company is nothing short of outrageous. It's a serious slap in the face to loyal and long serving staff - as well as the UK public," said GMB national officer Mick Rix.
"Taxpayer's cash has funded Genting to the tune of millions of pounds during lockdown - now they are making people redundant rather contribute towards the Government furlough scheme from the end of August."