James Harding, co-founder of Tortoise Media and former director of news at the BBC, said the corporation has got into a "muddle" over the issue of impartiality as he warned that the broadcaster cannot police the opinion of every contributor.

"I think it's part of a bigger muddle on impartiality," he told BBC Radio 4's Today programme.

"Why do we care about impartiality? We care about making sure that a publicly funded broadcaster that delivers news and information that informs the country is impartial, but people can make up their own minds on political issues.

"But you can't get to a world in which the BBC is policing the opinions of every writer, director, musician, sports personality, scientist, business entrepreneur.

"Not only can you not actually do it, but the principle is wrong.

"The principle is wrong because it will actually deter people from joining the BBC, it will diminish the BBC.

"But, even more importantly, there are freedom of speech principles here. Those people have lives beyond the BBC and should be able to give voice to what they say."