Her onscreen love affair with Matthew Crawley kept viewers on the edge of their seats for months, before the pair finally got married this series. But her real life relationship has broken down after five months.
Michelle, 30, is believed to have ended their relationship just a month after Joseph accompanied her to Los Angeles for the Emmy Awards, where she was nominated for best dramatic actress for her role as Lady Mary Crawley.
Joseph, 38, is said to be heartbroken over the split.
A friend of Joseph told The Sunday Mirror: "He's heartbroken. The split was unexpected and it's still very raw for him."
Joseph recently posted a series of cryptic tweets, including: "Just wait for someone worth being in a relationship with, stop wasting your time on the ones who don't deserve you."
Will you be tuning into Downton Abbey tonight (Oct 21)? 9pm, ITV1.
READ MORE: Downton Abbey actress Joanne Froggatt gets married
READ MORE: Downton Abbey's Julian Fellowes wants prequel about Lord and Lady Grantham