The BBC reportsthat Marsh has suffered a minor stroke and that scripts are being rewritten to accommodate her absence before filming starts next week.
Marsh, who plays maid Rose Buck, is the only cast member from the 1970s original series to appear in the rebooted version. She won Emmy nominations for her work in the role in 2011, 1974 and 1976, winning the award in 1975.
In a statement, she said "Unfortunately I won't be returning to Eaton Place quite as quickly as I had hoped. I am looking forward to spending time with Rose again when I'm match-fit and I miss the cast and crew tremendously."
The BBC wished Marsh a speedy recovery and said it had "something extra special planned for her new storyline".
'Upstairs Downstairs' executive producer Faith Penhale told the BBC that the cast and crew were looking forward to Marsh's return, adding that "Rose is a much loved, cherished member of the household who was instrumental in bringing everyone together in last year's successful revival."
News of Marsh's illness follows a shakeup at the drama. Marsh's co-creator Eileen Atkins quit over the summer, reportedly because she didn't like the direction the series was going in. She's been replaced by 'ER' and 'Doctor Who' star Alex Kingston, who'll be playing her character's much-younger racy half-sister.