A film about a black girl born to white parents in apartheid South Africa symbolises the "appalling" nature of the system, according to its star.

Sophie Okonedo said Skin summed up the "ridiculousness" of the country's former segregation policies as she attended the premiere in London's Leicester Square on Thursday.

Based on a true story, the film follows the life of Sandra Laing, who was born in the 1950s to Afrikaner parents - the difference in colour believed to be due to some kind of genetic throwback in their ancestry.

After being expelled from her all-white boarding school at the age of 10 and classified as "coloured", her parents, Abraham and Sannie, fought for her to be reclassified as white.

But Laing was later rejected by her father when at 16 she began a relationship with a black man.

The film, directed by Anthony Fabian, charts her struggle to find her place in a society torn apart by race and politics.

Attending Skin's UK premiere on Thursday night, Okonedo - dressed in a £5.99 dress from Oxfam - said: "I couldn't believe it was true. I was like 'are you sure?'

"It's such an extraordinary story and it's so symbolic of that time. It was appalling, the ridiculousness of the apartheid system.

"The story just shows it (the system) up for what it was."

Herself the daughter of a white mother and black father, the Rada-trained actress, who was nominated for an Oscar for Hotel Rwanda, added that attitudes in London were now more laid-back than in the 1970s when she was growing up.

Fri 3 Jul