The Independent says that Stephen Mitchell, deputy director of news at the BBC, told an audience at the University of Kent that Thompson's decision to sign a joint letter with non Murdoch national newspapers to Vince Cable, calling on the Business Secretary to intervene in the proposed takeover, compromised the ability of BBC journalists to cover an important story.
Mitchell said: "The BBC has to above all be impartial and almost as importantly be seen to be impartial in every issue of controversy in the UK. It is inevitable that we will cover the growth and role of News Corp going forward as part of our journalism.
"Mark Thompson is the editor-in-chief and I feel that that letter in a way compromises the perception of his impartiality on an issue of current controversy... For me, he compromises his role in life by signing a letter in the way that he did."
Mitchell, who was speaking at the Centre for Journalism at the University of Kent, and responding to a question from third-year journalism undergraduate John Saunders, was aware that his comments were on the record.
The BBC told the Independent that Mitchell "attended this event in a personal capacity and the views expressed were his own".