Disaster struck when the 20-year-old, who was wearing a short navy blue silk dress with plunging neckline, was en route to a pre-Baftas party.
A piece of tape which was supposed to be keeping the dress in place came undone, exposing more than she had intended.
She did her best to keep a straight face as the mishap was caught by cameras, even if the two girl friends she was travelling with didn’t quite know where to look.
Her daring costume is the latest sign that the star is keen to distance herself from her role as the uptight and nerdy trainee wizard Hermione Granger, which has turned her into a household name.
With only one more Harry Potter film to go, the star, who is reputed to be worth £19 million, has already began work on other projects.
