Dominic West's wife is said to be "shocked and devastated" over pictures of the actor kissing actress Lily James in Italy.
They were seen having an intimate lunch in Rome, where they kissed and cuddled.
And now a friend of Catherine Fitzgerald, Dominic's wife, believed she and him were "very much together" with a "good marriage".
The friend told MailOnline : "Catherine has seen the pictures and she's devastated. I came over to speak with her as soon as I knew about them.
"Catherine has been trying to speak with Dominic but he's not answering his phone. She's totally shocked because she didn't know anything was going on.
"They were very much together, so this is completely out of the blue.
"She thought they had a good marriage and now it's probably over. That's how she feels right now but the two of them need to speak but at the moment, she's lost for words."
Mirror Online has contacted reps of Dominic and Lily for comment.
Catherine and Dominic have been married since 2010 and share four children - daughter Dora, 13, son Senan, 12, son Francis, 11, and Christabel, seven.
Dominic's also dad to 22-year-old Martha, who he shares with ex-girlfriend Polly Astor.
Lily recently split from her Doctor Who star ex Matt Smith.
An insider told The Sun: "Lily and Matt were a picture-perfect couple for years but there are just some differences they haven't been able to get past.
"They have gone back and forth this year trying to sort things out between them because they both have so much love for each other.
"But unfortunately it just isn't working and they have mutually decided to just close this chapter of their lives."
Lily is perhaps best known for her role as Cinderella in Sir Kenneth Branagh's film about the titular princess, as well as in Mamma Mia's recent sequel.
Dominic captivated audiences in HBO hit The Affair, starring alongside Ruth Wilson.
The critically acclaimed series has run for five seasons.