A production crew has been at Shrewsbury's Dana prison for several weeks and the venue has now shut its doors to the public until November.
The Dana did initially stay open, with sections of the prison closed off to the public, but it is now completely closed aside from on weekends.
Neither the BBC, nor those behind the company that runs the prison tours, would confirm what production is being filmed but the crew is believed to be there for a new drama called Time.
The show had been due to be filmed in Liverpool but the city is now under the tight lockdown restrictions.
Joel Campbell, from the prison company, said: "All we can say is that the prison is closed to the public and there is a production taking place."
The former guards that show visitors around the Victorian building have also been tight-lipped about what series is being filmed.
Time is a three-part prison drama from Jimmy McGovern.
Seen through the eyes of two very different men, Time is described as a visceral and high-stakes portrayal of life in the modern British penal system. It is a story of guilt and forgiveness, punishment and penitence and the impact that prison has on all those who pass through it.
Sean Bean takes on the role of Mark Hebden, a teacher, husband and father, who killed an innocent man in an accident. Consumed by guilt, he accepts and even welcomes his four-year sentence.
Stephen Graham is Eric Reid, a prison officer whotries his very best to protect those in his charge.
When one of the most dangerous inmates identifies his weakness, Eric faces an impossible choice between his principles and his love for his family.
Both Eric and Mark, officer and inmate, are trapped by their pasts and unsure of their futures. Each has to not only survive, but to find the strength and the way to move forward.
Sean Bean said: “Getting to be involved in a Jimmy McGovern drama again is a real privilege and it will be great to be reunited with Stephen.
"Mark Hebden is another of Jimmy’s complex and superbly written characters and I am looking forward to bringing him to life on screen.”
Stephen Graham added: “It is an absolute honour to be working with Jimmy again, I’m feeling blessed. I’m excited to be working with Lewis as I have heard so many incredible things about him as a director, I can’t wait to get stuck in. Of course, I am made up to be back on set with Sean.”
It is not the first time the prison has been used as a filming location, with Holby City among the shows that have been shot there.