Varada Sethu (Doctor Who), Ethan Moorhouse (MobLand), Tuppence Middleton (Downton Abbey: A New Era), Merab Ninidze (Homeland) and Joshua McGuire (The Gentlemen) have all officially joined the show's cast for season 7, although details of their characters are yet to be revealed.
Ninidze appeared in season 6 in a minor voice role as KGB spymaster Vassily Rasnokov, so we can assume that he will be reprising that character.
Meanwhile, it has also been confirmed that James Callis will return as Claude Whelan, the former MI5 First Desk who left his role at the end of season 5 and hasn't been seen since.
News of Callis's return won't be a surprise to readers of Mick Herron's books, on which the series is based. Season 7 will adapt Herron's novel Bad Actors, in which Claude Whelan plays a central role.
The official synopsis for season 7 says that it "sees Lamb and his Slow Horses on the hunt to find and neutralise a mole at the heart of British Government before they can bring down the state".
Meanwhile, the blurb of the book, Bad Actors, says: "In MI5 a scandal is brewing and there are bad actors everywhere. A key member of a Downing Street think-tank has disappeared without a trace. Claude Whelan, one-time First Desk of MI5's Regent's Park, is tasked with tracking her down.
"But the trail leads straight back to Regent's Park HQ itself, with its chief, Diana Taverner, as prime suspect. Meanwhile her Russian counterpart has unexpectedly shown up in London but has slipped under MI5's radar. Over at Slough House, the home for demoted and embittered spies, the slow horses are doing what they do best: adding a little bit of chaos to an already unstable situation.
"In a world where lying, cheating and backstabbing is the norm, bad actors are bending the rules for their own gain. If the slow horses want to change the script, they'll need to get their own act together before the final curtain."
Want to see this content?
This page contains content provided by Google reCAPTCHA. We ask for your permission before anything is loaded, as Google reCAPTCHA may use cookies and other technologies. To view this content, choose 'Accept and continue' to allow Google reCAPTCHA and its required purposes.
We already also know that Gary Oldman will be returning as Jackson Lamb in season 7, but the rest of the returning cast has yet to be unveiled - after all, it likely depends on who makes it out of the upcoming season 6 finale alive.
Thankfully, the show is continuing to be produced at a pace, with an eighth season, based on Herron's book Clown Town, already confirmed.
Add Slow Horses to your watchlist on the Radio Times: What to Watch app – download now for daily TV recommendations, features and more.
Check out more of our Drama coverage or visit our TV Guide and Streaming Guide to find out what's on. For more TV recommendations and reviews, listen to The Radio Times Podcast.