The last instalment of Line of Duty's sixth series airs on Sunday and, while we're all ready to get to the bottom of Gail Vella's murder, long-time fans are hoping we'll finally learn the identity of mysterious OCG leader/corrupt senior police officer 'H'.

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First referred to in season three following the death of corrupt cop Dot Cottan, AC-12 have spent 18 episodes trying to uncover the last remaining bent copper who's high up in the Organised Crime Group – known by the codename 'H' or 'The Fourth Man'.

While Ted Hastings, Kate Fleming and Steve Arnott have managed to taken down Dot Cottan, Gill Biggeloe and Derek Hilton over the last six series, there's still one OCG-linked police officer at large – but who is 'H'?

Ahead of Sunday's finale, make sure to vote in RadioTimes.com's poll below to have your say as the nation waits with baited breath to find out who the secret criminal really is.

Could it be Chief Constable Phillip Osbourne (Owen Teale)? He's the Central Police boss revealed to have been working on the Lawrence Christopher case – which journalist Vella had been digging into before she was killed – and a keen advocate for merging the anti-corruption units and making redundancies.

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Similarly, viewers are suspecting Detective Chief Constable Andrea Wise (Elizabeth Rider), who is forcing Hastings into retirement and moving the spotlight away from institutional corruption, as well as Police and Crime Commissioner Rohan Sindwhani (Ace Bhatti), who seemed to have it out for Hastings before resigning in episode five – but could he be double buffing?

AC-3 boss Patricia Carmichael (Anna Maxwell Martin) certainly didn't cover herself in glory on Sunday after revealing she'd placed trackers on all AC-12 cars, replacing Hastings as the lead on their case and aggressively deflecting whenever 'H' or Philip Osbourne was brought up in the unit's interrogation of Jo Davidson – could she be 'H'?

Of course, we can't forget Marcus Thurwell (James Nesbitt), the SIO on the Lawrence Christopher case who's currently based in Spain (where 'H' is suspected to be located). While Spanish police believe they found him and his wife murdered in their villa, we don't know for sure whether it's definitely his body.

Then there's DCI Ian Buckells, though it's been tricky to tell whether he's fully corrupt – or just incompetent. Is he a pawn in someone else's game, or could he be 'H' himself?

We can never rule out Ted Hastings (Adrian Dunbar) and Kate Fleming (Vicky McClure) either, considering Hastings appears to have been responsible for the OCG discovering John Corbett was a rat and Kate's confusing behaviour in the last episode (why run from the scene when Ryan was killed in self-defence, Kate?).

Make sure to vote and let us know who you think 'H' or 'The Fourth Man' could be ahead of this weekend's finale. If you need a refresh on last Sunday's penultimate episode, read our Line of Duty episode six recap here.

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Line of Duty continues on Sundays at 9pm on BBC One. Take a look at the rest of our Drama coverage, or check out our TV Guide to see what’s on TV this week.

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