Grant will play a character named Simon Bricker who visits Downton Abbey as a guest of the Granthams.
Other new guest stars will include Anna Chancellor (The Hour) as Lady Anstruther and Rade Sherbedgia (Snatch) as a Russian refugee, Kuragin, who has fled the Russian revolution after the First World War.
Dame Harriet Walter will return as Lady Shackleton and Peter Egan is also back as Lord Flintshire.
Executive producer Gareth Neame commented: “We are delighted to welcome these talented actors to the world of Downton. The characters they play are set to bring yet more excitement and intrigue to the show.”
ITV has confirmed that Series 5’s eturning regular cast members are Maggie Smith, Hugh Bonneville, Michelle Dockery, Elizabeth McGovern, Jim Carter, Penelope Wilton, Phyllis Logan, Samantha Bond, Laura Carmichael, Lily James, Allen Leech, Brendan Coyle, Joanne Froggatt, Lesley Nicol, Sophie McShera, Rob James-Collier, Ed Speleers, Kevin Doyle, Raquel Cassidy, David Robb, Tom Cullen, Julian Ovenden, Daisy Lewis, Douglas Reith, Jeremy Swift and Andrew Scarborough.
Series 5 was announced by ITV last November and begins filming soon. Eight episodes will air this autumn, followed by a feature-length special at Christmas.
Are you looking forward to the Christmas special? Let us know below…