A Spanish influenza pandemic spreads through the Crawley family as the second series reaches its conclusion this weekend.
However, ITV has inadvertently revealed the identity of the departing character by releasing a full cast list for the third series on its website with one notable omission.
The broadcaster has previously confirmed that the period drama's leads Hugh Bonneville, Michelle Dockery and Maggie Smith would return next year.
Cora, the Countess of Grantham (Elizabeth McGovern), Lavinia Swire (Zoe Boyle) and Carson the Butler (Jim Carter) were all revealed to have the condition in last week's penultimate edition.
The Downton Abbey finale airs this Sunday at 9pm on ITV1.
> Ten teasers about the Downton Abbey finale