The Oscar-nominated actor is attached to feature in the latest big-screen installment of the period drama, which is due to begin shooting this summer ahead of a 2025 release.
Paul, 56, previously appeared in the 'Downton Abbey' TV series in the role of Harold Levinson and comes into the movie off the back of his acclaimed performance as the grumpy teacher Paul Hunham in 'The Holdovers' that earned him an Academy Awards nod - although he ultimately lost out to 'Oppenheimer' star Cillian Murphy.
An insider told The Sun newspaper's TVBiz column: "'Downton' fans will recognise Paul from the last time he starred in 'Downton Abbey' in the 2013 TV series, playing Harold Levinson, the brother of the Countess of Grantham. But getting him back is still something of a coup for the creative team because since then his career has seen him become even more in demand.
"That's only going to increase since he was nominated for his Best Actor Oscar. But he has now committed to being a part of the new 'Downton' film."
It was revealed earlier this month that a new 'Downton Abbey' flick was in development with stars including Hugh Bonneville and Michelle Dockery expected to shoot at Highclere Castle in Hampshire in the summer.
A source told The Sun: "This will be huge news for fans of the show who have remained just as addicted to 'Downton' long after it went from telly drama to movie blockbuster.
"There has been endless speculation about whether there would be a third movie and when it would be released, but finally devotees have had their prayers answered.
"The first two films proved such a success that a third one seemed inevitable, but the biggest problem was the logistics of bringing the cast back together as their diaries are all so packed."
'Downton Abbey' originally aired on ITV between 2010 and 2015 before moving to the big screen with a 2019 film of the same name and the 2022 sequel 'Downton Abbey: A New Era'.