Hollywood star Rhys Ifans has joined more than 200 actors calling for change at National Theatre Wales (NTW).
The 212 actors are demanding a commitment to cast more Welsh talent, and say larger audiences must be attracted.
The letter sent to chairman Clive Jones supported an earlier complaint by a group of 40 writers.
Mr Jones refuted some of the actors' claims and said NTW had a "Wales-first" casting policy.
Among those who signed the letter were Gavin and Stacey star Melanie Walters and Broadchurch's Matthew Gravelle.
The letter agreed with writers who have accused NTW of undermining Welsh artists, and said "many of us now feel a drastic disconnect from what once felt like a thriving and engaging theatrical forum".
The actors said they supported the writers' demands that:
- There should be Welsh or a Wales-based "primary artist" in all shows
- Non-Welsh artists and companies should be "engaged only to support a Welsh or Wales-based artist"
- Every show should include "theatre"
They also asked NTW to invest in opportunities to expand the talent pool, along with longer runs, larger audiences and an "ability to engage an international audience."
Mr Jones said NTW believed in "open-minded, plural and expansive theatre" that was internationalist but "rooted in Wales".
"The company welcomes this dialogue, but will not hesitate to rebut factually inaccurate claims," he said.
"The suggestion that the company's output has drastically diminished in the last two years is an example of such inaccuracy. During the leadership of the company's founding artistic director (2008-2015), NTW staged 41 productions.
"During the tenure of current artistic director Kully Thiarai (since 2016), the company has staged 21 productions in just over two years."
He added: "NTW will always strive to offer roles first and foremost to Welsh and Wales-based performers."