The broadcaster has already announced the casting of Daniel Mays who will play Sergeant Danny Waldron, a member of an Armed Response Unit which comes under scrutiny from the AC-12 team.
Mellor will play PC Rod Kennedy, Ali (The Missing, Silent Witness) PC Harinderpal ‘Hari’ Bains and Leanne Best (Ripper Street, Fortitude) WPC Jackie Brickford.
The four join returning cast Martin Compston as Detective Sergeant Steve Arnott, Vicky McClure as Detective Constable Kate Fleming and Adrian Dunbar as Superintendent Ted Hastings.
Series writer and creator, Jed Mercurio, said: “Sergeant Danny Waldron appears to run a team of crack firearms officers, but hidden tensions simmer beneath the surface.
“We’re all hugely excited that Will Mellor, Arsher Ali and Leanne Best have agreed to play these pivotal roles.”