Index on Censorship and English PEN have called for answers after 'Homegrown', a play about radicalisation, was cancelled by the National Youth Theatre shortly before its first performance.

The play was due to be staged by a cast of 112 young people aged between 15 and 25, and was to tackle issues around extremism and radicalisation, including why so many young British Muslims feel drawn to the Islamic State.

In a letter published in the Times on 14 August, leading writers, artists and actors joined free speech organisations in warning of a "culture of caution" that was shutting down debate and discussion.

There are also concerns, shared by the National Secular Society, about the exact series of events which led to the play's cancellation ten days before it was to be performed, and the signatories write of the troubling "reports that the NYT [National Youth Theatre] may have been put under external pressure to change the location and then cancel the production.

"Police, local authorities and arts organisations have a duty to respect and protect freedom of expression — even, and most especially, where they disagree with the message or find it controversial.

"We urge the NYT to give a full account of what led to the decision, and hope that a way can be found to stage it so that the young voices involved can be heard and the production can be judged on its merits."

Signatories include David Aaronovitch and Jodie Ginsberg of Index on Censorship, Maureen Freely, the president of English PEN, and Shami Chakrabarti, the director of Liberty.

Index on Censorship previously described the cancellation as "very worrying" and argued that "as a society we should be encouraging vehicles that shed light on the processes of radicalisation among young people. The way forward is to open up discussions about controversial subjects in contemporary Britain."

Stephen Evans, campaigns manager at the National Secular Society, commented: "It is concerning that the details of what exactly led to the play's cancellation are still unclear.

"Discussion about radicalisation and Islamic extremism should not be off-limits, and we hope that misplaced sensitivity over religion was not behind the cancellation.

"Reports about 'external pressure' placed on the organisers are very troubling and there needs to be a full account of what took place. Free speech must be defended, whether encroached on by the Government and police, or through inappropriate deference to religious feelings."

Meanwhile, a planned Mohammed Cartoon exhibit has been cancelled with organisers citing security concerns. In a statement on the ShariaWatch website, Anne Marie Waters said "the risk of running this exhibition is simply too high".