The 46-year-old actor is famous for playing Americans in series such as Homeland and Billions. In order to perfect his twang, he keeps it going away from the set, but as soon as he's around his daughter Manon, 10, and nine-year-old son Gulliver, he reverts to his British accent.

And the fear that the children might start speaking in an American manner is something Damian shares with wife Helen MCCrory.

"I don't do it in front of them," Damian told Mr Porter magazine. "They once spent a school term in the US and they started copying the accent in this comical, exaggerated way, elongating their vowels. I put a stop to that. My wife has a horror that the children will start talking American if we spend too much time out there."

Portraying Nicholas Brody in Homeland and Bobby "Axe" Axelrod in Billions means that speaking in an American accent has become "second nature" to Damian. However, he admits his friends back in England are less than impressed when he keeps his twang going while not filming.

"I once did a US TV interview in my American accent and somehow it was seen by quite a few of my friends back in England," he laughed. "There were several irate text messages. 'You are British! Stop talking like an American!' It has become second nature to me now and I am happy to do it. That said, if I am with Brits I come out of it pretty quickly."

Showtime series Billions, in which Damian stars as Axelrod, an ambitious hedge fund manager who builds his fortune from insider trading is currently airing its second season. But filming in New York isn't all fun and glamour.

"A show like Billions means extraordinarily long hours," he moaned. "You get a 60-page script every week to 10 days. You have to get learning it immediately. You are doing 60-70 hour weeks.

"The turnover is relentless and everyone is working flat out. There's no time for a beer when you've wrapped for the day because you are exhausted," he sighed.