Her performances as Downton’s Lady Mary have earned Michelle Dockery fame, fans and even award nominations on both sides of the Atlantic. Now her sister has revealed the rather childish origins of her success — claiming her take on the character is simply the “posh lady” impression she honed during her childhood in Essex.

Jo Dockery, 35, who is also an actress, said it had come as no surprise at all to her that Michelle, 31, had become a star — as she had been doing impressions since she was just five years old.

“The only difference is that now everyone knows about it. I’ve always known what she can do,” Jo told Red magazine.

“She was always doing impressions. It’s actually funny watching Downton [Abbey] because Lady Mary is basically Michelle’s ‘posh lady’ impression when we were little.”

The two sisters grew up with a third, Louise, 37, in Romford and attended the Finch Stage School, in Essex, from a young age.

As a girl, Michelle developed a stutter as she struggled to get a word in against her chatterbox siblings.

But now the sisters said they were close and even hoped to work together on a new comedy series, written by Jo, which Michelle aims to produce and possibly act in.

Jo, who is to join the cast of The Curious Incident of the Dog in the Night-Time, said: “It’s about our childhood, about family, and a lot about stage school.

“One of the characters is an amalgamation of all the teachers and choreographers I’ve worked with.”

Michelle said they were keen to get their sister Louise involved, too. “The best times are when we’re together,” she said.

“We’re so close and we really support each other. It’s the Essex Mafia.”

The Golden Globe and Emmy-nominated actress said their upbringing meant she and her sisters remained down to earth.

“The way we were brought up means our expectations are never too high, so everything is like an amazing surprise.

“Mum’s favourite saying is, ‘Now, don’t get your hopes up,’ which sounds like a negative thing to say but it has helped me a lot. She taught us to enjoy every day.

“I’m in this amazing show and really enjoying it. Why should I think beyond that? ”

The full interview appears in the October issue of Red, on sale September 4.