The actor, who plays DI Steve Arnott in the hit BBC cop show, has said the “natural story arc that we’ve been on for the last six years” is coming to an end and the cast will not return “for the sake of it”.
Speaking on The Jonathan Ross Show, Martin said he has the future of the show lies in the hands of creator and writer Jed Mercurio.
“That’s nothing different for us,” he said. “Jed always takes months after... there’s stuff above our pay grade, analytics, figures and all that kind of thing that comes in.
“He always takes time off. But I think it is important to say, I think this natural story arc that we’ve been on for the last six years (is coming to an end).
“We won’t come back just for the sake of it. That’s for sure.”
He added: “We’ll come back if there’s a story to tell. But, so as well, if it ends well maybe sometimes it is best to leave it. But, so, genuinely we don’t know.”
A trailer for Sunday’s series finale has promised viewers that the identity of ‘the fourth man’ – otherwise known as ‘H’ – will be revealed.
AC-12 boss Superintendent Ted Hastings is among the suspects, along with DCS Patricia Carmichael, Chief Constable Philip Osborne, DI Kate Fleming and ex-officer Marcus Thurwell.
The internet is already flooded with theories ahead of the finale, as many fans believe Marcus Thurwell is actually alive, after the character was found dead at his Spanish villa – and some of then claim to have the proof.
Suspicions have also been raised about DCS Carmichael, as some viewers spotted what they believe might be a clue that confirms she’s ‘H’.
The series has been a huge hit for the BBC, with the penultimate episode bringing in an average of 11 million viewers in the overnight ratings.
Some 15.2m have now also watched the first episode, according to consolidated figures that take into account viewers that watched on catch-up services and other devices in the 28 days since it aired.
The Jonathan Ross Show airs on Saturday at 9.35pm on ITV, while Line Of Duty concludes on Sunday at 9pm on BBC One.