“But this is life. Dan felt that this was right thing for him, and the right moment to move on to different areas.”
Lord Fellowes, who won an Oscar for his screenplay of Gosford Park, disclosed he would have written an alternative ending for Matthew’s character had circumstances been different.
“If he [Stevens] had been prepared to come back for maybe two or three episodes in a series, that would be different. Then we could have had a foreign posting or invented a career that would have made it possible for him to be away.
“Otherwise we would have had to make this tremendously successful love affair between Mary and Matthew unhappy, which I didn’t feel would be believable.
“For them to then separate and Matthew never set eyes on his son again would not be believable either.
“So we didn’t really have any option. By him dying, their love can remain in tact.”
He emphasised there had been no “fall out”, saying: “I’m pretty sure he’s got a terrific future. We will miss him.”
Lord Fellowes, who said he found the popular notion that writers and producers made all the decisions about scripts “charming”, added: “I’m rather amused by the idea that these plot decisions are taken by producers and writers rather than the actors.
“In truth, they are taken entirely by the actors."
Despite his reluctance to kill off the heir to Downton, Lord Fellowes promised viewers there would be no miraculous Dallas-style re-appearance for Matthew.
“Matthew will be dead,” he joked. “Mary will not find Matthew in the shower; that I can promise.”
The makers of Downton Abbey have already moved to console devastated viewers about the "untimely and tragic death" of character Matthew Crawley, after an outpouring of furious complaints about the Christmas special.
Viewers complained it was "travasty", a "tragedy" and an "outrage", with some saying the ending had "ruined their Christmas".
The makers of the programme yesterday released a statement explaining the decision, reassuring fans the death was the only credible exit for the heir to Downton.
His "solid and loving marriage" to Lady Mary had meant a separation would be inconceivable, they said.
The explanation follows a barrage of complaints about the plot, which saw character Matthew Crawley killed in a motoring accident as he drove to share news of his happy wife and newborn baby.
Within moments of the episode being broadcast, thousands of distressed viewers took to social networking sites and forums to complain about the ending.
One accused the writers of "killing my soul" while another said the plot "shattered my heart". A third noted: "I AM 100% DONE WITH THIS SHOW. I CAN'T BREATHE."
Others were seemingly too distraught to speak, saying: "I don't even want to discuss what happened in the Downton Abbey Christmas special. I just don't."
One mournful fan wrote "I love Downton Abbey but they have just made me very angry", as another claimed: "Downton Abbey you have just ruined my Christmas Day!"
Some attempted to put on a brave face, saying: "I'm still not over the Downton Christmas Episode, so I'm just going to rewatch all the happy episodes and pretend everything is swell."
A spokeswoman for ITV, which broadcasts the award-winning programme, last night issued a statement to explain the decision to kill the character off and comfort fans.
Saying actor Dan Stevens had chosen not to renew his contract beyond the initial three years, she said the "solid and loving marriage" between characters Matthew and Mary has left writers with no choice about his inevitable departure.
The relationship between the two was so strong that they could not "simply be estranged or parted", she said, meaning he had to be dramatically killed off in the surprise twist.
Speaking of his departure exclusively to the Daily Telegraph, Stevens said he had chosen to leave the popular show after the second series to pursue "a desire for freedom".
"So there is a strange sense of liberation at the same time as great sadness because I am very, very fond of the show and always will be," he said.
The death of Matthew Crawley in a motoring accident is the second unexpected twist in Downton Abbey this series, with Lady Sybil passing away following childbirth in emotional scenes earlier this year.
The two surprise deaths have already led to speculation about the future of the show, with some fearing it will struggle to recover from the loss of two popular characters.
Last night, a spokeswoman for ITV said: "After three successful series and two Christmas editions of Downton Abbey, Dan Stevens decided not to renew his contract beyond the initial three years he had been contracted.
"We wish him every success for the future. Michelle Dockery will be returning to her role as Lady Mary in series four which begins production in February.
"Over the last three years, audiences across the world have been captivated by the ups and downs of Mary and Matthew’s relationship, culminating in their wedding.
"Fans have enjoyed what has become a solid and loving marriage. It is for this reason that the Producers decided Matthew and Mary could not simply be estranged or parted, resulting in his untimely and tragic death at the end of the Christmas episode.
"In the next series, alongside all the usual drama, comedy and romance involving the much loved cast of characters, viewers will see Mary adjusting to her life and attempting to move on without the man she loved."
So, now we have a little more behind the scenes knowledge. It doesn't make it any more palatable. But it makes Downton endearing to me again. And so as I turn the corner in my fictitious grief, I think of what Season 4 has in store for all of the Crawleys.