The government has announced a phasing out of its furlough scheme and confirmed it will end in October - which BECTU has warned will lead to "mass redundancies" in theatre.
Chancellor Rishi Sunak also announced an extension to its Self-employment Income Support Scheme, which industry bodies have welcomed.
The coronavirus job retention scheme has allowed businesses to claim 80% of wages up to £2,500 for employees who are put on paid leave, called furloughing.
In June and July the scheme will continue as before.
It will then be phased out in the following steps:
• From August employees will have to pay National Insurance and pension contributions
• From September the government will pay 70% of wages and employers will pay 10%
• From October the government will pay 60% of wages and employers will contribute 20%
The scheme will then close in October.
Additionally, the government is introducing flexible furloughing from July, which will allow employers to bring back workers on a part-time basis by paying wages for days worked.
Responding to the announcement, BECTU head Philippa Childs said: "The changes to the CJRS will lead to mass redundancies in theatre, film and TV, as many employers won’t be prepared to contribute for someone who won’t work for them again in the near future or they simply can’t afford the contributions.
“The requirement to pay National Insurance and pension contributions to access the CJRS will be too difficult for theatres to shoulder.
"TV and film workers on PAYE contracts - de facto freelancers – will also be hit. It is highly unlikely they will be returning to the employer who has furloughed them, and, as a result the employers concerned may cease to offer any assistance from that point."
Chief executive of the Creative Industries Federation, Caroline Norbury, said the job retention scheme had been a "lifeline" for the industry.
She said: "The ability to furlough part-time earlier than planned is welcome, however clarity is needed beyond October. As called for in our statement with UKHospitality and Association of Leading Visitor Attractions yesterday, it is imperative that we avoid a cliff-edge on these vital government support schemes, ensuring relief continues for those businesses who will take longest to resume full levels of operation."
Sunak also announced that the Self-employment Income Support Scheme has been extended by three months, with applications for a second and final grant opening in August.
The grant will be worth 70% or up to £2,570 of average monthly profits, reduced from 80% paid in the first three months of the scheme.
Childs said "overall, the measures announced today will not help many workers in the creative industries who have been largely ignored throughout this crisis".
“Many of our members have had no access to income support throughout this crisis and still have no prospect of returning to work. Despite the government trying to provide for freelancers during the pandemic, ultimately it has failed as too many of them fall between the gaps of the SEISS and the Coronavirus Job Retention Scheme," she added.
Meanwhile, Equity said the months ahead would be "very difficult" for people who "continue to be excluded from SEISS, including the most vulnerable and under-represented workers in our industry", such as graduates and those who rely on PAYE to supplement their income.
Christine Payne, general secretary, said the chancellor needed to "work with us to develop a plan to protect the creative industries".
"At the core of a recovery plan for the creative industries there must be an income guarantee for the self-employed and freelance workforce. We cannot hope to return to work in full until the early months of 2021 and this represents a massive threat to the £111bn we contribute to the economy," she added.