text: She will play Denker, a lady's maid to Dame Maggie Smith's character, the Dowager Countess of Grantham.
  Johnston, who has also appeared in Waking the Dead and Coronation Street, joins new stars Richard E Grant and Anna Chancellor, both of whom will play guests of the Granthams at Downton.
  The hit period drama will return to screens this autumn.
  Series four of the show, which followed the wealthy Grantham family and their servants, achieved an average of 11.9 million viewers in the UK.
  The very British drama has also been a huge hit in the US, winning both Emmy Awards and Golden Globes.
  More than 26 million viewers watched series four on Masterpiece on PBS, making it one of the highest rating shows on American television.
  Previous high profile guest stars include Shirley Maclaine who played Martha Levinson, Lady Grantham's mother, and Oscar-nominated actor Paul Giamatti who appeared in last year's Christmas special as her "maverick, playboy" son.
  Series five will also feature 24 star Rade Sherbedgia as a Russian refugee who has fled the revolution after World War 1.
  Earlier this year, executive producer Gareth Neame promised it would have "all the usual highs and lows, romance, drama and comedy".
target: The Royle Family actress Sue Johnston is the latest star to join series five of ITV's Downton Abbey in a guest role.