On Tuesday, Maestro – the BBC’s educational page dedicated to tutorials from big names – featured a glimpse of the major actor in a clip and photo included on the website.
The pic, which appears in a forthcoming episode, depicts Detective Inspector Steve Arnott (Martin Compston) looking over a database of characters.
One of the faces visible in the database reportedly belonged to the well-known new cast member, whose involvement in Line of Duty had previously not been announced.
A short scene involving the star was also available to view on the site.
After each episode of the drama is aired, BBC Maestro shares excerpts from the show, with Jed narrating his thought process behind the plot.
But this week, users were able to access the course trailer for next Sunday’s episode — including scenes featuring the new actor and a crucial twist.
Addressing the gaff, Jed made light of the spoiler, telling The Sun: “The outstanding detective work of eagle-eyed Line of Duty fans has been awarded a glowing commendation from the gaffer himself, Supt Ted Hastings.
“Their call-up papers for AC-12 are in the post!”
Another familiar face is set to return to the BBC drama in this Sunday’s episode.
It has been announced that fan favourite Anna Maxwell Martin is returning as AC-3 boss Superintendent Patricia Carmichael.
Anna appeared as the no-nonsense officer in the final two episodes of series five, quickly becoming a hit with viewers.
She led the interrogation of AC-12′s Superintendent Ted Hastings when he was under suspicion of being mysterious organised crime boss “H”.
Carmichael will make her reappearance this weekend, with bosses hinting she could be about to take control after Hastings was forced into retirement, following Chief Constable Philip Osborne’s plan to cut numbers in Central Police’s anti-corruption units.
Of rejoining the show, Anna said: “Pat’s back, I can’t remember what she’s doing there, nor would I be allowed to blab, but suffice to say she’s got some new clobber and she’s ready to bust some balls.”
Line Of Duty continues on Sunday at 9pm on BBC One.