More than a million Facebook users in Britain could have had their personal data accessed by Cambridge Analytica, the company revealed on Wednesday, as it increased its estimates of the total number of users affected from 50 to 87 million.

Mark Zuckerberg, the embattled chief executive of the social media giant, refused to rule out legal action against the British company and insisted that he remained the best man to lead the tech firm, flatly denying he had been asked to resign.

In a rare teleconference with reporters, Mr Zuckerberg, 33, sidestepped questions as to why he had declined to appear before a committee of British MPs, pointing out that earlier in the day it was confirmed that he would testify before US politicians.