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Former Downton Abbey actor Hugh Bonneville suspected his late mother was a spy for MI6, he has told the ´óÏó´«Ã½.

Revealing his Desert Island Discs on ´óÏó´«Ã½ Radio 4, the actor, who also starred in ´óÏó´«Ã½ comedy W1A, said she never talked about her three decades of work for the security service.

When he asked her outright, she denied being a spy, he told the programme.

He also confesses to having felt intimidated acting with his on-screen mother in Downton, Dame Maggie Smith.

He told Kirsty Young his parents met when his father was training as a doctor and his mother as a nurse. She followed him to Singapore and worked for the Foreign Office there

When he was about 10 years old and the family were living in London, his mother took a job for three days a week.

But he did not ask her about it until much later on.

He said: "We used to drop her off at her office sometimes at Lambeth North.

"I opened the newspaper one day and it said 'Century House MI6 building to be sold' and I looked at the photograph and said, 'Mum, that's your office.'

"And she said 'Mmm, yes dear.' I said, 'You're a spy?'.

"She said 'No, I'm not a spy dear.'"

Bonneville said his mother died just over a year ago. When he asked his father if she ever said anything about her work: "He said, 'Never, she just went to the office.'"

"All I know is she didn't have special umbrellas or knives coming out of her toe caps or anything like that, she did just work in the office.

"But I am extremely proud, not only that she found fulfilment in that work, as well as bringing up us kids, but that she never spoke about it."

Her nickname in the valley where his parents retired, he said, was "the Colonel", as she was "a do-er".

Speaking about the Dowager Countess, his character Lord Grantham's mother, played by Maggie Smith, in ITV's Downton Abbey, he said he was "terrified" by her high-calibre acting.

He said: "I can remember the very first scene I did with her and I was absolutely terrified, and I think I can remember the last scene with her and I was absolutely terrified.

"She is the most astonishing actress. Her wit is legendary, as you say, and she doesn't suffer fools. And you raise your game, you have to.

"It's great acting opposite her because she's got extremely high standards for herself and expects them of others."

Downton Abbey ran for six series, with the last episode screened in the UK on Christmas Day 2015.