Sunday night’s episode of the BBC One series saw DSU Ian Buckells, played by Nigel Boyle, unmasked as the mysterious H – the corrupt police officer at the top of a criminal conspiracy.
Fans and critics were split over the reveal with some saying they felt let down and others praising the decision to avoid a more dramatic conclusion.
However Mercurio says that audience research tells a better picture than the online snark of social media.
"No-one disputes the Line of Duty finale divided social media opinion but the audience research so far shows a far less extreme picture," the show creator explained on Twitter.
“We knew a "down" ending would rate less favourably with some viewers," he concedes, before going on to explain that the final episode was rated only slightly lower by viewers than the first episode of the season.
Explaining the audience data, he added: "The research determines the episode ratings based on randomly polling viewers, rather than sites like imdb where scores can be skewed by users strongly motivated to register their immediate anger/adulation …”
"1000 random viewers submitted scores from 1-10 which have been used to calculate the Appreciation Index (AI) as a score out of 100. The AI for the “down” finale was only 7 points below the next lowest in Season 6 (Ep 1) …”
As writer of the show, which first hit TV screens on BBC Two in 2012, Mercurio is no stranger to divided opinions surrounding the show’s storylines and accepted that the debate surrounding the finale is “all part of the experience of shared TV viewing.”
“These figures won’t stop the debate, of course, nor should they - that’s still all part of the experience of shared TV viewing.”
After it was announced that the finale episode broke UK viewing records with 12.8 million viewers, Mercurio acknowledged the accolade and the mixed viewer response:
"We're honoured and flattered by the viewing figures for #LineOfDuty6."
"We knew attempting to explore the real nature of corruption in our society wouldn't appeal to everyone, but we do sincerely thank you all for watching."
"It's been our privilege to share your Sunday nights."
Actor Martin Compston, who plays DI Steve Arnott in the series, also tweeted in response to the ‘down ending.’
“Fully understand it wasn’t the ‘urgent exit’ type ending some anticipated but we appreciate you sticking with us,” he shared on Twitter.
The much-anticipated finale was watched by an average of 12.8 million people, with a 56.2% share in overnight viewing figures, making it the most watched episode of a drama in 20 years, the BBC said.
Martin Compston, who plays Steve Arnott, revealed that he, Adrian Dunbar (Ted Hastings), Vicky McClure (Kate Fleming) and Mercurio had agreed to get “AC12million” tattoos if the show hit the milestone – in reference to the show’s crime-busting AC-12 unit.
Watch: Mercurio says H hint was dropped in S1 of Line of Duty