Two writers claim they were removed from a high-profile theatre production about to open at the Manchester international festival and were threatened with legal action if they spoke out.

In a blogpost titled Tree. A story of gender and power in theatre, Sarah Henley and Tori Allen-Martin claim they conceived the show, which is billed as a creative collaboration between the actor Idris Elba and the director Kwame Kwei-Armah.

After four years working on the project they were dropped, they said, and then went through eight months “where we questioned everything and felt bullied and silenced”.

They continued: “For us it’s been devastating proof of the way doors are shut on women, and on the underdogs. We became completely disposable because we’re not famous or important enough. We were expected to shut up, lie down, and take it.”

Tree, which is a fusion of music, drama and dance, is being touted as one of the highlights of the festival, opening on Thursday before a run at London’s Young Vic in August.

Elba’s Green Door Pictures company, the Young Vic and the festival (MIF) said in a joint statement they were “deeply saddened” to read the online article. All three were “passionate about supporting and nurturing emerging talent within the creative industry from the widest variety of backgrounds” and were “committed to ensuring fair representation on stage and behind the scenes”.

They said they appreciated that Henley and Allen-Martin “were involved in exploring ideas for a project based on Idris’s original concept”, but “the truth of the matter is that MIF and Green Door did not feel their proposed direction was artistically viable. It was decided by these producers that the show needed to go in a very different direction with a new writer attached, using Idris Elba’s original concept as the starting point.”

The added: “Several offers were made to Tori Allen-Martin and Sarah Henley to discuss the future of the show, and how the producers could continue working with them, which they declined.”

Henley and Allen-Martin, writing on the online publishing platform Medium, said the show was based on their work. “We put four years of work into that project, and the majority of those involved read our script, our proposal documents, our premise, and our synopsis – there is no way it’s a ‘different project’, no matter how much it’s changed.”

The two writers said they had consulted lawyers and decided it would be too expensive to take the case to court. They said Tree’s producers had “continued to make financial offers to ‘buy us off’”.

The show’s producers rejected that characterisation, saying offers were made in the spirit of reaching a compromise. They also denied a breach of any legal obligation, saying “Despite attempts by the producers to resolve the issue with them, they decided to instruct solicitors.”

Henley and Allen-Martin said the most pressing issue was paying legal fees of £5,000 and have set up a donations page online to raise the money. Anything above that £5,000 figure will go towards a new organisation the pair have set up called Burn Bright, with the aim of driving positive change for female writers.

Kwei-Armah, who succeeded David Lan at the Young Vic after seven years running Center Stage in Baltimore, tweeted his personal response to the claims and said he hoped the disagreements could be debated at a public discussion with Henley and Allen-Martin.

He expressed sympathy for the two writers but rejected their version of events. “I understand the pain of being ‘released’ from a project. It has happened to me, and it cuts deep.”