Michelle Obama has criticised Facebook executive Sheryl Sandberg's "lean-in" feminist mantra, telling an audience "that s**t doesn't work all the time".

The former First Lady made the comments at an event in Brooklyn, New York to promote her new memoir Becoming, according to Vanity Fair.

It was an unusual critique of Mrs Sandberg, who wrote a book entitled Lean In in 2013 that encourages women to get ahead in the workplace by "leaning in" to their ambitions.

Mrs Sandberg argued that women could "have it all" if they were assertive in taking up leadership roles and worked hard enough.

However, Facebook's chief operating officer herself later admitted that the philosophy has not been as effective as she hoped.

Mrs Obama, whose own book broke sales records in its first two weeks, dismissed the idea that women can "have it all" and insisted marriage is still "not equal".