Female employees who 'cover up' at work were more likely to be promoted instead of women who wear provocative clothes such as tight miniskirts or low cut tops.
According to a poll of 1,000 women, even Eighties-style "power dressing" is no way to earn a promotion.
But although 61 percent thought the biggest faux pas was excessive cleavage, the survey voted Holly Willoughby the best look for the modern British office.
The 31-year-old TV presenter, who won the Best Celebrity Cleavage award earlier this year, received 35 percent of the votes.
Her runner-up, 'Harry Potter' star Emma Watson was some way behind, having been chosen by 15 percent of those questioned.
The poll, which sought to uncover how attitudes to female fashion at work have changed, found the worst thing women can do at work is overdress.
More than half of those surveyed (54 percent) frowned at tight mini-skirts at the office and 51 percent said slapping on excessive make-up was a strict no-no.
"The office is the new catwalk and fashion doesn't stop during office hours," the Daily Express quoted Julie Donnelly, head of womenswear buying at the fashion website Very.co.uk, as saying.
"Instead of trying to make a power statement or impress 'the boys', the easy glamour exhibited by Holly Willoughby, individuality and not trying too hard can only be a good thing," she added.