Kelly Macdonald has revealed the "top cast" of Line Of Duty know who H is ahead of the series finale but said even people who work on the show are being kept out of the loop. 

Macdonald, who plays the guest lead in the latest series Joanne Davidson, also said she did not know about a pivotal twist about her character’s parentage until halfway through filming.

Series six of the BBC One crime drama comes to an end on Sunday, but a number of characters remain under suspicious in the hunt for the “the fourth man” – an individual commanding a network of corrupt officers behind the murder of journalist Gail Vella.

The penultimate episode of the police procedural, created by Jed Mercurio, earned an average of 11 million viewers and 51.7% of the audience share, according to the broadcaster.

The sixth series of the BBC One show, starring Compston as DI Steve Arnott alongside Vicky McClure, Adrian Dunbar and Kelly Macdonald, will come to an end this weekend (2 May). A seventh series has not yet been announced.

The show’s creator, Jed Mercurio, whose credits also include Bodyguard and Bodies, said: “Thanks so much to everyone watching #LineofDuty6. The whole production team is thrilled and flattered by the amazing response.”

Macdonald, 45, described Mercurio as “troublesome” when speaking to the PA news agency.

She said: “Jed Mercurio, man, he is troublesome. He keeps a lot to himself. I feel like Martin (Compston, co-star) always heard about things first. I felt like he would get the lay of the land.

“I don’t know if that was just because he is so on top of everything and understands everything acutely and what the connotations are.

“I didn’t have the full story on Jo. I just knew that things hadn’t been great for her. Jed kept saying, ‘She is this enigmatic figure’ and he was being enigmatic himself.

“By the time we came back after the first lockdown I knew what was up.”

Speaking about which stars know the identity of H, she added: “The main cast know. I don’t know about everybody. H knows who H is… Like I said, I don’t know the whole cast, but I know the top cast.”

The Trainspotting star admits she has become more “reclusive” since the start of series six.

“It has just been odd. I can’t tell you how odd it has all been. I just got told the numbers from (last Sunday) and it is a lot,” she said.

It’s just a lot of people watching it and I am still in the living room, not going online and not watching. It feels like a slightly different dimension.”

The Line of Duty series final airs on BBC One and iPlayer on Sunday at 9pm.