IRISH author and Line of Duty superfan Marian Keyes led the social media backlash against the controversial ending which has divided the more than 12 million viewers who tuned in to see the mystery revealed.
Keyes tweeted: "Like, NO!!!!!!!!!!!!!!!!!! I've NEVER felt so LET DOWN!!!!!! #LineOfDuty."
Broadcaster and author Piers Morgan used a reference to a previous clue with a spelling error in it, as he tweeted: "Definately a tad underwhelming. #LineofDutyFinale."
Critics were impressed though, with The Times' Ben Dowell giving it four stars out of five, writing: "Perhaps Mercurio has more surprises in store. It's the sort of thing he does. Roll on series seven I say."
The BBC One police procedural drama has steadily increased its fan base as people speculated on the identity of the mystery character.
Charlotte Moore, the BBC's chief content officer, said: "Addictive event television, Line Of Duty has kept the nation guessing for the last seven weeks, so it's no surprise that last night's jaw-dropping finale set a ratings record.
"Jed Mercurio is a master of his craft, and I would like to congratulate him and the entire cast and crew for delivering such an incredible drama series. I'm looking forward to having a conversation with the team about where we go next and what the future of the series might be."
There was much hilarity at the fact that the surprise appearance of Co Antrim actor James Nesbitt as a potential candidate for `H' proved to be a red herring.
In the event he appeared only in a holiday snap and a mocked up police HR photograph, leading to amused speculation on social media about his payday and potential BAFTA award for acting.
Nesbitt's cameo was widely agreed to have been writer Jed Mercurio's main `gotcha' of Series 6, playing into the intense speculation which followed his promise that the central mystery would finally be revealed.
