The successful corporate lawyer said she was a timid teenager but got on by 'faking it'.
She added: "If you don't have it, you fake it.
"Women have been faking lots of things during history. No? So if you're going to fake something, fake self-confidence.
Story continues after the video
In an interview with the Evening Standard, mum-of-three sons Miriam also warned that girls were dropping out of 'hard' subjects like Maths and English for fears they are 'not going to be attractive to boys'.
She made headlines earlier this month while discussing her role as a working mother, saying: "I don't want to have it all - I want to have what men have".
The Telegraph reports that Miriam and Nick want to 'always keep' their three boys in state school, and that Miriam said that attending 'Eton' was not a shortcut into the elite.
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