Exclusive - Unforgotten creator teases Nicola Walker's replacement for season 5

Speaking exclusively to RadioTimes.com, Lang also confirmed for the first time that the unnamed character and new 'partner in crime' to DI Sunny Khan (Sanjeev Bhaskar) will definitely be another woman.

Asked about whether he had begun working on the new lead role, he says, "I certainly have begun conceiving the character, yeah. I'm actually working at the moment. And I had yeah, [the] script was commissioned before we even aired season four.

"So I've kind of got a rough idea, obviously, who she is. It's an evolutionary process when you write both the series and the characters and they evolve over the course of the writing process, six months or so. So I have a pretty good idea who she was and who she is and what her story is. And yes, it's a woman. But it will keep evolving until the day we deliver it because you still rewrite in the [series] edit."

He also confirmed that the female lead will be "a new character," and won't be recruited from the existing team who worked with Cassie.

"It is a new character, yeah. It's not one of the existing team, although they will remain," he says.

Lang (who recently penned the second series of Innocent) also opened up about the death of DCI Cassie Stewart during season four, revealing that he and Nicola Walker discussed her departure while they were in the middle of making season three.

"It was decided between me and Nicola, really. It was a discussion that we had together over a couple of series. In fact, we certainly discussed it when we were making series three. So it was just a mutual decision that her story would probably come to an end by the end of season four."

Was there debate about whether to leave the door open for her return in future seasons? "I don't think so. No, because I think we both felt it was the right decision and you have to be firm with yourself about that," Lang says.

"It would have been easy to write it in such a way that she could have come back at some point, but I don't think she, Nic, would have thanked me for that. I wouldn't have thanked myself if that's even possible. Because you have to be bold and say we're moving into a new iteration of the show - and don't give yourself an out to return to a safer version of it. Because moving into season five, the worst thing I could do is become complacent and to relax, really, and I need to keep setting myself challenges and holes that I need to get out of.

"And I think it will, you know, having a new character alongside Sunny Khan will give it a different energy and you always need to be looking for that every single season."

All four series of Unforgotten are available to stream on the ITV Hub. Visit our TV Guide to discover what else you can watch tonight, or find all the latest news on our Drama hub.