The Borgias star, 29, attended Britain's prestigious Marlborough College, alongside the likes of Catherine, Duchess of Cambridge and her sister Pippa Middleton, but she hated the idea of her affluent peers finding out about her dad's job.
She tells Britain's The Times, "I never told people who my parents were. It was my idea of hell. At one school I told everyone he was a lawyer. I didn't want to be separate. I wanted to be like everyone else. I still struggle with it. It's fine, it's part of who I am. It's nice not to be known as 'the daughter of'. I haven't found it's opened too many doors and even if it did, you still have to be good."
However, Sumner, whose mother is film producer Trudie Styler, insists she was never ashamed of rumours surrounding his lengthy tantric sex sessions.
She adds, "It was massively blown out of proportion. It was a joke, which became the biggest, stupidest story everyone became obsessed with. I definitely wasn't embarrassed, but it wasn't something we discussed around the dinner table."