Mr Zuckerberg, co-founder and chief executive of the US technology giant, broke his silence to head off a revolt among users and financial backers. He described the incident as a "major breach of trust" and said he was open to further regulation of the website.

ISBA, a British group of advertisers that spend hundreds of millions of pounds a year on Facebook, demanded answers. It is understood that some of its 3,000 brands, which include those of the consumer goods companies Unilever and P&G, will not tolerate association with Facebook if it emerges that users' data has