Kelly Macdonald has joined the cast of 'Line of Duty' as a major new guest character.

Line of Duty

Line of Duty

The 'Trainspotting' star will play Detective Chief Inspector Joanne Davidson, the senior investigating officer of an unsolved murder, whose unconventional conduct raises suspicions at AC-12.

'Line of Duty' showrunner Jed Mercurio has promised that Kelly's alter ego will be an "enigmatic adversary" for Vicky McClure, Martin Compston and Adrian Dunbar's characters in the BBC One police drama.

Jed said: "We're honoured that Kelly Macdonald will join Martin, Vicky and Adrian in Series Six of 'Line of Duty'. DCI Joanne Davidson will prove the most enigmatic adversary AC-12 have ever faced."

Piers Wenger, Controller of BBC Drama, added: "We're delighted that Kelly Macdonald is coming on board for series six. She's one of the UK's most versatile actors and we're excited for viewers to see what Jed's scripts have in store for her when she joins the Line of Duty team."

'Line of Duty' Series Six will start shooting next year and as well as Kelly's casting confirmation more guest stars are expected to be announced in the coming weeks.

Writer Jed recently revealed that he hopes the next series will be aired "sometime in 2020", but he admitted there is no confirmed air date yet.

He said: "I would hope its sometime in 2020, next year. But that's not something we're able to confirm."

The series follows AC-12's Superintendent Ted Hastings (Adrian), DI Kate Fleming (Vicky) and DS Steve Arnott (Martin) as they investigate corrupt police officers.