Contestants on the BBC quiz show Mastermind will no longer be allowed to pick Fawlty Towers, Blackadder or Roald Dahl as their specialist subjects as the producers have run out of questions.

Mark Helsby, Mastermind’s producer, has revealed that some subjects are so popular that they have been “exhausted” for new questions. Father Ted and the Chronicles of Narnia have also been removed.

Since Mastermind and its famous black chair was launched in 1972, it has become renowned as one of the toughest quiz shows in Britain, with contenders answering questions on a chosen subject before a general knowledge round. The programme was initially hosted by Magnus Magnusson but the questions are now asked by John Humphrys, who also presents Radio 4’s Today programme.

In a rare insight into the workings of the programme, Helsby revealed that Harry Potter was the most requested specialist subject last year – with 262 people wanting to answer questions about the wizard series when only one of them could – and that there must be “at least two good quality books” on a biographical or historical subject, while one book or one film is not allowed.

“Thirty-two people wanted to do Fawlty Towers last year,” Helsby told the Radio Times, explaining why some subjects would no longer be an option. “19 wanted Blackadder and 22 wanted Father Ted. Some of the very popular literary subjects such as the Chronicles of Narnia or Roald Dahl probably wouldn’t be agreed to for the same reason.”

Helsby said some topics were too narrow, such as Romeo and Juliet, or too broad, such as the entire works of Shakespeare, and that Mastermind does not repeat subjects from one series to the next.

“You can do any subject you want – within reason. They have to be suitable for broadcast on BBC Two on a Friday night, but pretty much everything is doable, provided there’s sufficient source material to refer to,” he said.

The producer also addressed criticism of Mastermind that it is dumbing down by allowing contestants to answer questions about modern television when a rival is addressing a historical subject such as Leonardo da Vinci.

“People have been accused of taking the easy option by choosing a comedy like Friends or Frasier but they have over 200 episodes each, so that’s still a lot of research,” Helsby said.

“There’s sometimes a debate on Twitter about whether it is fair to have a ‘classic’ Mastermind specialist subject in the same programme as someone doing a sitcom, but we treat equally and it’s the skill of the question writers, verifiers and the team putting the questions together in the office to make sure that they are comparable tests of knowledge.”