Bectu chief Philippa Childs said at their conference in Brighton: "The public deserve better, not a reduction in democratic access to a public service broadcaster."
A spokesman for ATG said: "Edinburgh Playhouse generally pays its staff rates of pay that are above
Bectu negotiated levels and has done so for many years."
BECTU successfully turned the spotlight on low pay at the BBC as the Corporation defended pay deals with its highest paid on-screen talent.
A BBC Scotland spokesman said: '
BECTU, who we have a very good working relationship with, have raised a number of issues with us on behalf of members who work on River City.
BECTU claims the BBC has said the move is necessary to "even out" production between London and Birmingham.
The dispute centres on
Bectu's claim for a PS300 daily rate for "grips", a specialist grade in camera departments, affecting big budget drama output produced by the BBC, ITV and the independent sector.
Gerry Morrissey, general secretary of
Bectu, said: "Our members are suffering because the BBC thinks it can deliver the same levels of output with many fewer staff.
Bectu general secretary Gerry Morrissey said, "We would prefer to have a sensible conversation with BBC management about the damage done in the first year of these cuts, but instead we've had to turn up the pressure to protect thousands of members from over-work, bullying and stress.
Bectu assistant general secretary Luke Crawley said: "There is no doubt in my mind that this outcome has only been achieved because of the readiness of union members to stand together and show their collective strength in support of a valued colleague.
Ms Williams, a producer with 20 years experience, is the long standing chair of the
Bectu branch involved in negotiations with management.