Who is the prison guard in Line of Duty? Everything you need to know as Alison Merchant returns once again

Actress Maria Connolly plays the formidable recurring character, officially named Alison Merchant, who was first seen in series two where she brutally attacked DI Lindsay Denton (Keeley Hawes) with boiling water.

She later lied on her report to suggest that Denton had poured the scolding liquid over herself, but eventually was replaced as AC-12 brought the case to a close.

She returned midway through series six to intimidate Farida Jatri (Anneika Rose) into silence, breaking her wrist while she was in her cell awaiting an interrogation from Hastings and his team.

She reared her head once again in the penultimate episode of the series, where she not-so-subtly revealed that she has her sights set firmly on Jo Davidson (Kelly Macdonald).

The copper, who is in custody both for her misconduct at work and for the killing of PC Ryan Pilkington (Gregory Piper), was being guided to her cell when Merchant appeared with her corrupt associate.

They claimed to have taken a wrong turn in the maze-like institution, but it was quite clear they were really sending Davidson a warning that they would soon be coming after her too.

AC-12 have placed Davidson, revealed to be both the daughter and the niece of deceased crime boss Tommy Hunter, in a cell protected by CCTV – but there's no guarantee that will withstand the seemingly unlimited reach of the dreaded OCG.

Given everything we've seen so far, it seems highly likely that Merchant is on the OCG payroll, so it's crucial that AC-12 root her out of her position before she can meddle with their investigation any further.

Line of Duty continues on Sundays at 9pm on BBC One. If you are enjoying the new series, you can read our Line of Duty episode 6 recap here. Also take a look at the rest of our Drama coverage, or check out what else is on with our TV Guide.