And co-star Vicky McClure claimed she is ready to go through with the promise after the finale of season six of the BBC police corruption drama was watched by 12.8 million people on Sunday evening.
Compston addressed fans on Twitter after reaction to the identity of the corrupt police officer known as 'H' — who the AC-12 team have been hunting since the first series in 2012 — caused some disappointment when it was finally revealed to be minor character DCI Ian Buckells, played by Nigel Boyle.
The 36-year-old Scottish actor — who has played DI Steve Arnott in all six series of the show — tweeted: "Hey folks now the madness is slightly dying down, just want to thank everybody who reached out about last night and the series as a whole, we've truly been blown away by the reaction and love for the show we're proud of it. Fully understand it wasn't the 'urgent exit' type ending some anticipated but we appreciate you sticking with us.
"Felt many emotions from excitement and anxiety at the hype to relief that it's over. Now I just feel immense pride our show that started on Wednesdays on BBC2 has just smashed every overnight drama record since they began."
He added: "Finally myself, Vicky, Adrian and Jed made an agreement in the inconceivable and ridiculous scenario we ever got twelve million viewers we would get 'AC12million' tattoos. We'll see who's good to their word..."
He is referring to fellow original cast members McClure - who plays DI Kate Flemming - and Adrian Dunbar, 62, who plays Superintendent Ted Hastings, along with Jed Mercurio, the writer and creator of the hit crime drama.
McClure, 37, replied: "We are so grateful to you all. So much so looks like I’ll have the ink to prove it!"
Mercurio has retweeted her message in apparent agreement.
Dunbar is not on social media and is yet to comment as to whether he will be getting permanently inked in honour of the show.
Despite smashing its ratings record with the season finale - up 3.5m viewers on season five - the last episode has been widely criticised for being an anti-climax.
Line Of Duty has followed the investigation of insiders in the police force working with criminal organisations over the past five series. Their long-running hunt for 'H' has always led viewers to believe the identity would be that of someone very senior within the police force and possibly even a major cast member within the AC-12 team.
While Boyle has played Buckells since the first series, he has always been a very minor role who slipped under the radar, and so his exposure as 'H' left many disappointed.
However, Mercurio has hinted there may be another series of the show.
Read more: LoD's Martin Compston admits he and Vicky McClure are always pranking on set