Fans of Downton Abbey can expect the series to take a dark turn, thanks to a “harrowing” new storyline centred around Thomas Barrow.

Rob James-Collier, who plays the sometimes sinister under-butler, admitted that he has been suffering sleepless nights over the script, which sees his character question – and try desperately to change – his sexuality.

“There are mornings of those scenes when you haven’t been able to sleep at night because you are so worried about how it will go,” James-Collier told the Daily Mail.

“Thomas comes right up to the abyss.”

“It’s quite a harrowing storyline and I hope it is. Because if it isn’t, I haven’t done it justice.”

His quotes come just days after Hugh Bonneville – who plays Robert, Earl of Grantham in the show – said he believed the show could have another five years left to run.

Alan Leech (Tom Branson) and Rob James-Collier (Tom Barrow) on the set of Downton Abbey in August 2014

“This is all written by one person and there’s only so much one person can do,” he said.

“There will be an end. (Executive producer) Gareth Neame has said he reckons it will run between five and ten years, so we’ve made it to halfway.

“I think there will be natural end and I’m sure Julian will want to quit while he’s ahead.”

Downton Abbey series 5 Show all 103 1 /103 Downton Abbey series 5 Downton Abbey series 5 Downton Abbey: Season 5 Mr Bates and Anna behind bars ITV Downton Abbey series 5 Downton Abbey: Season 5 Atticus with a prostitute ITV Downton Abbey series 5 Downton Abbey: Season 5 Andy, Mr Carson, Molesley and Thomas ITV Downton Abbey series 5 Downton Abbey: Season 5 Above and below stairs gather for the WWI memorial ITV Downton Abbey series 5 Downton Abbey: Season 5 Atticus and Lady Rose, who wears a large engagement ring ITV Downton Abbey series 5 Downton Abbey: Season 5 Lady Sinderby, Lord Sinderby and Atticus following his engagement to Lady Rose ITV Downton Abbey series 5 Downton Abbey: Season 5 Lady Mary Crawley, Master George Crawley, Baxter, Cora, Countess of Grantham, Denker, Lady Edith Crawley, Mrs Hughes, Violet, Dowager Countess of Grantham, Mrs Patmore, Tom Branson, Miss Sybbie Branson and Isobel Crawley ITV Downton Abbey series 5 Downton Abbey: Season 5 Mr Carson and as Robert, Earl of Grantham at the memorial service ITV Downton Abbey series 5 downton11.jpg Anna lines up as part of a police identity parade ITV Downton Abbey series 5 Downton Abbey: Season 5 Mr Bates at the WWI memorial service ITV Downton Abbey series 5 Downton Abbey: Season 5 Mrs Patmore and Daisy in the kitchen ITV Downton Abbey series 5 Downton Abbey: Season 5 Cora, Countess of Grantham ITV Downton Abbey series 5 Downton Abbey: Season 5 Lady Mary and Lady Edith looking happy together ITV Downton Abbey series 5 Downton Abbey: Season 5 Violet and Isobel Crawley talking at the ceremony ITV Downton Abbey series 5 Downton Abbey: Season 5 The Dowager Countess' new maid Denker chats with Mr Bates ITV Downton Abbey series 5 Downton Abbey series 5 Edith returns from London ITV Downton Abbey series 5 Downton Abbey series 5 Love is in the air for Lady Rose and Atticus ITV Downton Abbey series 5 Downton Abbey series 5 Allen Leech as Tom Branson ITV Downton Abbey series 5 Downton Abbey series 5 Isis is sick ITV Downton Abbey series 5 Downton Abbey series 5 The boys: Lord Merton, Tim Grey, Robert, Earl of Grantham and Atticus Aldridge ITV Downton Abbey series 5 Downton Abbey series 5 The girls: Lady Rose, Lady Mary and Mabel Lane Fox ITV Downton Abbey series 5 Downton Abbey series 5 Aspiring cook Daisy, played by Sophie McShera ITV Downton Abbey series 5 Downton Abbey series 5 Can Mabel Lane Fox (Catherine Steadman) convince Tony Gillingham (Tom Cullen) that she's the woman for him? ITV Downton Abbey series 5 Downton Abbey series 5 Maggie Smith as the Dowager Countess ITV Downton Abbey series 5 Downton Abbey series 5 Lady Mary continues to look pensive. Is she any closer to deciding upon a lover? ITV Downton Abbey series 5 Downton Abbey series 5 Lord Merton (Douglas Reith) joins the Grantham family at the Abbey ITV Downton Abbey series 5 Downton Abbey series 5 There's a new face in the form of Lord Merton's son, Larry Grey, who is played by Charlie Anson ITV Downton Abbey series 5 Downton Abbey series 5 Lily James as Lady Rose ITV Downton Abbey series 5 Downton Abbey series 5 David Robb as Dr Clarkson and Robert James-Collier as Thomas Downton Abbey series 5 Downton Abbey series 5 Tensions are high between Cora and Robert after his confrontation with Simon Bricker Downton Abbey series 5 Downton Abbey series 5 Elizabeth McGovern as Cora, Countess of Grantham Downton Abbey series 5 Downton Abbey series 5 The cast of Downton Abbey indulge in some racing at a Point to Point Downton Abbey series 5 Downton Abbey series 5 Michelle Dockery looks like she means business as Lady Mary Crawley Downton Abbey series 5 Downton Abbey series 5 Catherine Steadman as Lord Gillinghams former lover Mabel Lane Fox Downton Abbey series 5 Downton Abbey series 5 Lady Rose and Atticus strike up a friendship in Downton Abbey series 5 Downton Abbey series 5 Downton Abbey series 5 Thomas, Molesley and Mr Carson serve drinks in Downton Abbey series 5 Downton Abbey series 5 Downton Abbey series 5 Robert James-Collier as under-butler Thomas Downton Abbey series 5 Downton Abbey series 5 The Downton Abbey children, George and Sybbie play with cuddly toys Downton Abbey series 5 Downton Abbey series 5 Michelle Dockery as Lady Mary Crawley and Oliver/Zac Barker as Master George Crawley Downton Abbey series 5 Downton Abbey series 5 Cora, Violet and Isobel in Downton Abbey series 5 Downton Abbey series 5 Downton Abbey series 5 The Dowager Countess and her former lover Kuragin share an intimate conversation Downton Abbey series 5 Downton Abbey series 5 Lady Rose is all smiles as she talks to Charles Blake Downton Abbey series 5 Downton Abbey series 5 Matt Barber as Lady Rose's new love interest Atticus ITV Downton Abbey series 5 Downton Abbey series 5 Michelle Dockery plays Lady Mary in Downton Abbey series 5 episode 5 ITV Downton Abbey series 5 Downton Abbey series 5 Underbutler Thomas Barrow in Downton Abbey series 5 ITV Downton Abbey series 5 Downton Abbey series 5 Richard E Grant plays art historian Simon Bricker ITV Downton Abbey series 5 Downton Abbey series 5 Cora, Simon Bricker and Molesley look ominous at Downton's door ITV Downton Abbey series 5 Downton Abbey series 5 Sophie McShera plays maid Daisy, who tries to better herself by learning maths ITV Downton Abbey series 5 Downton Abbey series 5 Howard Ward plays Sergeant Willis ITV Downton Abbey series 5 Downton Abbey series 5 Thomas returns downstairs to speak to Daisy in episode five ITV Downton Abbey series 5 Downton Abbey series 5 A sullen Lady Mary sits next to her father Robert, Earl of Grantham ITV Downton Abbey series 5 Downton Abbey series 5 Louis Hilyer plays Inspector Vyner ITV Downton Abbey series 5 Downton Abbey series 5 Richard E Grant as Simon Bricker and Elizabeth McGovern as Cora, Countess of Grantham ITV Downton Abbey series 5 Downton Abbey series 5 Catherine Steadman as Mabel Lane Fox, Julian Ovenden as Charles Blake, Michelle Dockery as Lady Mary Crawley and Samantha Bond as Lady Rosamund ITV Downton Abbey series 5 Downton Abbey series 5 Daisy Lewis plays schoolteacher Sarah Bunting in Downton Abbey ITV Downton Abbey series 5 Downton Abbey series 5 Lord Gillingham and Lady Mary Crawley have a heated discussion in Downton Abbey series 5 ITV Downton Abbey series 5 Downton Abbey series 5 Michelle Dockery as Lady Mary and Samantha Bond as Lady Rosamund in Downton Abbey series 5 ITV Downton Abbey series 5 Downton Abbey series 5 Peter Egan returns as Lady Rose's father Shrimpie ITV Downton Abbey series 5 Downton Abbey series 5 Richard E Grant flirts with Cora in the role of art historian Simon Bricker ITV Downton Abbey series 5 Downton Abbey series 5 Lady Mary and Robert, Earl of Grantham have a serious-looking discussion in the Downton grounds ITV Downton Abbey series 5 Downton Abbey series 5 Lady Mary, Robert, Earl of Grantham and Tom Branson look ready for business ITV Downton Abbey series 5 Downton Abbey series 5 Penelope Wilton as Isobel Crawley in Downton Abbey ITV Downton Abbey series 5 Downton Abbey series 5 Robert James-Collier as Thomas in Downton Abbey ITV Downton Abbey series 5 Downton Abbey series 5 Christopher Rozycki as Rostov and Rade Sherbedgia as Kuragin. ITV Downton Abbey series 5 Downton Abbey series 5 Michelle Dockery as Lady Mary Crawley, Allen Leech as Tom Branson and Hugh Bonneville as Robert, Earl of Grantham study a map ITV Downton Abbey series 5 Downton Abbey series 5 Prince Kuragin, Isobel and Violet, Dowager Countess of Grantham take a look into the past ITV Downton Abbey series 5 Downton Abbey series 5 Michelle Dockery dresses up to the nines as Lady Mary Crawley in Downton Abbey Nick Briggs Downton Abbey series 5 Downton Abbey series 5 Rade Sherbedgia as Kuragin, Elizabeth McGovern as Cora, Countess of Grantham, Lily James as Lady Rose, Maggie Smith as Violet, Dowager Countess of Grantham, Hugh Bonneville as Robert, Earl of Grantham and Michelle Dockery as Lady Mary Crawley Nick Briggs Downton Abbey series 5 Downton Abbey series 5 Michelle Dockery as Lady Mary Crawley in Downton Abbey Nick Briggs Downton Abbey series 5 Downton Abbey series 5 Lady Mary Crawley is all smiles ahead of her saucy weekend with Lord Gillingham Nick Briggs Downton Abbey series 5 Downton Abbey series 5 Jeremy Swift as Spratt in Downton Abbey series 5 Nick Briggs Downton Abbey series 5 Downton Abbey series 5 Howard Ward as Sergeant Willis and Brendan Coyle as John Bates as the case of valet Green comes under scrutiny again Nick Briggs Downton Abbey series 5 Downton Abbey series 5 Richard E Grant as Simon Bricker and Elizabeth McGovern as Cora, Countess of Grantham admire some artworks Nick Briggs Downton Abbey series 5 Downton Abbey series 5 Richard E Grant as Simon Bricker and Elizabeth McGovern as Cora, Countess of Grantham Nick Briggs Downton Abbey series 5 Downton Abbey series 5 Simon Bricker charms Cora in Downton Abbey Nick Briggs Downton Abbey series 5 Downton Abbey series 5 Robert, Earl of Grantham is left 'red faced and furious' when things do not go as he had planned Nick Briggs Downton Abbey series 5 Downton Abbey series 5 Robert James-Collier as Thomas on the telephone in Downton Abbey Nick Briggs Downton Abbey series 5 Downton Abbey series 5 Rade Sherbedgia as Kuragin and Christopher Rozycki as Rostov Nick Briggs Downton Abbey series 5 Downton Abbey series 5 Raquel Cassidy as Baxter Nick Briggs Downton Abbey series 5 Downton Abbey series 5 Cora, Countess of Grantham, Elizabeth McGovern) and Simon Bricker (Richard E Grant) deep in conversation ITV Downton Abbey series 5 Downton Abbey series 5 Lady Edith with her estranged daughter ITV Downton Abbey series 5 Downton Abbey series 5 Socialist school teacher Sarah Bunting (Daisy Lewis) and friend of Branson, creates tension at Downton ITV Downton Abbey series 5 Downton Abbey series 5 Mrs Patmore (Lesley Nicol) and Daisy (Sophie McShera)are wondering what the future holds amid the changing social and political landscape ITV Downton Abbey series 5 Downton Abbey series 5 The household appears transfixed by a gramophone... ITV Downton Abbey series 5 Downton Abbey series 5 ...but Carson does not look impressed. ITV Downton Abbey series 5 Downton Abbey series 5 Mr Carson, played by Jim Butler, and Mrs Hughes (Phyllis Logan). The loyal Carson is struggling to adapt to the changing times ITV Downton Abbey series 5 Downton Abbey series 5 Under-butler Thomas Barrow, played by Rob James-Collier. The actor has told audiences to expect a "harrowing" storyline centred around his character's sexuality Downton Abbey series 5 Downton Abbey series 5 John and Anna Bates (Brendan Coyle and Joanne Froggatt) in the fifth series of Downton Abbey - how will their marriage hold out? ITV Downton Abbey series 5 Downton Abbey series 5 Sophie McShera as aspirational household cook Daisy ITV Downton Abbey series 5 Downton Abbey series 5 For Isobel Crawley, played by Penelope Wilton, romance could be on the cards Downton Abbey series 5 Downton Abbey series 5 Elizabeth McGovern as Cora, Countess of Grantham, and Richard E Grant as Simon Bricker ITV Downton Abbey series 5 Downton Abbey series 5 Anna Chancellor as newcomer Lady Anstruther Downton Abbey series 5 Downton Abbey series 5 Lady Edith (Laura Carmichael) is struggling to deal with the fallout from her illegitimate pregnancy... Downton Abbey series 5 Downton Abbey series 5 ...and can't keep away from her secret child Downton Abbey series 5 Downton Abbey series 5 Will Lady Mary (Michelle Dockery) pursue a romance with Lord Gillingham (Tom Cullen)? Downton Abbey series 5 Downton Abbey series 5 Mary's stepbrother (Allen Leech) could also be in line for a new love... Downton Abbey series 5 Downton Abbey series 5 ...in the form of socialist school teacher Sarah Bunting, played by Daisy Lewis Downton Abbey series 5 Downton Abbey series 5 Jimmy (Ed Speleers) is the subject of Lady Anstruther's infatuation Downton Abbey series 5 Downton Abbey series 5 Harriet Walter will reprise her role as Lady Shackleton ITV Downton Abbey series 5 Downton Abbey series 5 Lady Edith (Laura Carmichael) with her secret child Marigold... ITV Downton Abbey series 5 Downton Abbey series 5 ...and with the child's adoptive parents, Mr and Mrs Drewe ITV Downton Abbey series 5 Downton Abbey series 5 Emma Lowndes plays the farmer's wife ITV Downton Abbey series 5 Downton Abbey series 5 Richard E Grant as Simon Bricker ITV

Fellowes himself has said that “the most important thing is to make sure Downton is well-formed, comes to an end at the right time, and is not lured into the usual thing of trying to keep something going past its time”.