When it comes to baring all for the sake of art they don't come any more experienced than actress Helen Mirren.

She's never shied away from stripping off, so who better to show more bashful actresses how to do it?

That's exactly the position the forthright 58-year-old found herself in for her latest movie, Calendar Girls, which tells the true story of WI women who shed their clothes to raise money for charity.

Mirren is joined naked on screen by some of Britain's most respected actresses, including Julie Walters and Annette Crosbie, and was more than happy to help them lose their inhibitions.

"I was a bit like the cheerleader," she smiles.

"I had to be because it was easy-peasy for me - I've been filmed naked before. But it was tough for the other actress who'd never done nude scenes.

"Annette Crosbie is nearly 70 years old and has never had to appear nude in a career stretching more than 50 years, but she turned out to be the coolest of us all.

"By the time those nude scenes came along we'd all been together for a long time and we were a good group and we loved and supported each other and did as much as we could to make each other feel great. Ultimately, it was an experience that no one regretted or felt bad about."

With an enviably slender figure belying her age, Mirren was more than happy to strip off again after receiving the script for Calendar Girls.

Mirren was impressed by the tale of Yorkshire women who set out to raise money for the Leukaemia Research Fund by posing nude for a Women's Institute calendar.

"I just remember reading about them in the newspaper and thinking, 'Wow, what a great story. How funny, how cool'."

She was even more eager to get involved in the movie when she was told her character was based on Tricia Stewart, the woman who originally came up with the idea for the nude calendar.

And despite their personality differences, when Hollywood superstar Mirren met Yorkshire WI member Stewart they instantly clicked.

"I don't look like her, and I'm not like her but when I met her I realised Tim (writer Tim Firth) had tapped into her dynamism and energy," explains Mirren of her alter ego.

"She has such a big personality, she's spontaneous and has a spirit that gives her courage, commitment and passion about what she does."

Although her life couldn't be more removed from the WI scene, Mirren admits she became fascinated with the quintessentially British organisation, during the making of the film.

"I didn't realise how eccentric it can be. Women are extraordinary because they're so talented and imaginative and sometimes they put those talents and imagination into such funky, surreal channels."