Rumours of Stephen joining the cast began circulating last year after a blurred image of the This Is England and Boardwalk Empire star was shared online.
Last year Kirkby born Stephen received acclaim for his role in the ITV drama Little Boy Blue and has also played crime kingpin Al Capone in Boardwalk Empire.
He has now been shown in character alongside guest star Sandall, with the pair looking threatening in their roles as John Corbett and Lisa McQueen.
The picture released by the BBC shows the new characters looking stern, surrounded by a gang of associates in their organised crime group.
They are at the command of a mysterious figure known only as “H”.
Sandall recently starred in Silent Witness so she is no stranger to a police procedural.
Stephen previously said of his casting: “It’s an honour to have been asked to be part of the Line Of Duty team.
“I’m especially looking forward to working with Martin Compston and Vicky McClure again.”
The pair will be joining series regulars Compston, who plays DS Steve Arnott, McClure as DS Kate Fleming and Adrian Dunbar in the role of Superintendent Ted Hastings.
Penned by Bodyguard writer Jed Mercurio, Line Of Duty will return to BBC One with a six-part series later this year.
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