Inspector Morse creator Colin Dexter wrote a clause in his will banning anyone else playing the part of the detective after his death – to prevent future actors 'competing' with John Thaw.
The author died in 2017 and a memorial service was held at Christ Church Cathedral in Oxford the following year.
Before he died he made it clear that he did not want any other TV dramas to follow Endeavour in the series - Inspector Morse, Lewis and Endeavour was enough.
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The author said: "We never want to repeat what John has done.
"A lot of people connected with Morse didn’t want anyone coming along to say we will try and outdo dear old John. I said I’m not ever going to allow that, full stop.”
The existence of the clause was revealed in an interview with the Radio Times by Shaun Evans, who has played a young Morse in Endeavour.
The producers of the series only managed to convince the author to consent to Mr Evans in the role as he would not be competing with John Thaw’s more mature original.
“Shaun will be the last person to play Morse,” Mr Dexter said. “That was made clear from the word go.”
It is expected that Mr Dexter’s wishes will be enforced by his estate, who retain the rights for 70 years after his death.
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The author wrote 13 novels with Morse as the protagonist.
The first, Last Bus to Woodstock, was published in 1975 and the series finished with The Remorseful Day almost 25 years later when the character died of a heart attack.
John Thaw starred in the TV adaptation Inspector Morse from 1987 until 2000. He died in 2002.
Shaun Evans was cast as a young Morse in 2011. The production company wanted to make a one-off film for the 25th anniversary of Morse on television and it was popular enough to be commissioned as a series.
It took Michele Buck, a joint managing director of Mammoth Screen, four years to persuade Mr Dexter to back the new show.
“I convinced him, saying the clause in his will wasn’t because he was worried about someone playing a 27-year-old Morse; he just didn’t want anyone competing with John Thaw,” she said. “He believes John was perfect for the part.”
Mr Dexter said: “He (Evans) is about the age Morse would have been when he came up to St John’s Oxford and I wish him well; he very much deserves it.
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“We didn’t want someone to imitate John Thaw, his movements or the way he spoke. We wanted someone free and natural in the part and someone who loved it.”
In his interview with the Radio Times, Shaun Evans revealed that rather than mimic Thaw, he settled on a Monty Python star for the detective’s mannerisms.
He said: "I listened a lot to Michael Palin, who was from the North, went to Oxford and was alive at that time. That’s how I imagine Morse’s voice."
Mr Dexter was given several cameo roles in Inspector Morse, Lewis and Endeavour.
He was awarded an OBE for services to literature in 2000.
A year later, he was awarded the Freedom of the City of Oxford, an honour awarded to those who have given eminent service to the city.
The Endeavour series started in 2012. Series nine - the last began on Sunday and finishes on March 12.
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This story was written by Andy Ffrench, he joined the team more than 20 years ago and now covers community news across Oxfordshire.
Get in touch with him by emailing: Andy.ffrench@newsquest.co.uk
Follow him on Twitter @OxMailAndyF
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