Why is Tosh leaving Shetland and who could replace her? Viewers devastated after fan-favourite departs the show

Shetland fans are united in grief over the departure of fan-favourite character DI ‘Tosh’ McIntosh, as actress Alison O'Donnell says goodbye to the series for ever after season 11.

Tosh wasn't a major character in Ann Cleeves' original Shetland novels on which the show is based, and Alison was a guest star when the show premiered in 2013.

Appearing alongside original lead Douglas Henshall (DI Jimmy Perez), Tosh was such a hit that the character was promoted to series regular.

When Douglas left and was replaced by Ashley Jensen's DI Calder, not only was Tosh promoted to Detective Inspector, Alison's chemistry with Ashley helped make the change of leading role a smooth one, and fans loved the pairing of the leading ladies.

But why is Tosh leaving? When the new season begins airing on November 12, viewers will see how the character's exit storyline plays out after Alison O'Donnell handed in her notice and left to take on other projects.

The actress tells Radio Times, "I've been involved with it for 14 years now... at this point it feels like we've probably explored it enough. I don't think I would ever have found a moment to step away that didn't feel sad and bittersweet, but something in me said 'leap', and so I did".

Filming her final scene was a devastating moment for the star, who shares, "Our producer Angus [Sharpe] made a speech after we wrapped my final scene but sadly I can't really remember what he said because I was crying, it just came over me."

She adds, "It just felt so overwhelming. I think it's going to take me years to fully process it."

The writing appears to be on the wall that Tosh isn't happy in her job at the beginning of season 11. After the season 10 finale that saw her uncover PC Billy McCabe's (Lewis Howden) bribe-taking, their friendship is in tatters at the start of the new episodes, leading to what Alison calls "an unholy cocktail of awkwardness and tension" that makes her reconsider her role.

When she's later involved in scenes involving "the building of special sets, an incredible stunt artist, and some extremely deep emotional digging" to film, this sets the scene for her eventual departure.

Our lips are sealed beyond this, as full details of what happens are strictly under wraps, but Alison has publicly revealed, "thoughtful scenes with all the important people" will play out across what will be a "memorable exit."

Tosh's departure of course leaves a vacancy at Lerwick police station, and while her replacement hasn't been confirmed, there's speculation surrounding who could be taking up Alison's reigns.

Names being thrown around include Ralf Little, the former lead of Death in Paradise who starred as DI Neville Parker in the series. Martin Clunes is another favourite, who has become known for portraying no-nonsense professionals living in tight-knit communities.

Douglas Henshall has also been speculated to make a return to the show, but this could be more wishful thinking from fans than anything else - his character arc seemed to come to a solid conclusion and a return seems unlikely.

Of course, there's a chance Tosh might not be replaced at all, but an existing character could get a promotion.

DC Sandy Wilson (Steven Robertson) is another character who has been with the series from the beginning, and the longest-serving remaining detective could step into the role of Calder's partner.

There's also the chance that supporting officers like PC Lorna Burns (Eubha Akilade) and PC Alex Grant (Conor McCarry) could be given the opportunity to flex their investigative muscles and take on bigger roles in the wake of Tosh bidding farewell - you'll have to watch this space.

Season 11 of Shetland airs on BBC One and BBC iPlayer from November 12.