Boris Johnson said he has never heard of BBC presenter Naga Munchetty, who has been at the centre of controversy over on-air comments about US president Donald Trump.
The corporation's director-general Lord Hall was forced on Monday night to overturn a decision made by the BBC's Executive Complaints Unit (ECU), which had ruled that she had breached editorial guidelines.
READ MORE: Thousands sign petition to overturn BBC's ruling on Naga Munchetty
During an interview with LBC Radio, the Prime Minister said he had "never heard of her" when asked by host Nick Ferrari if he thought she had indeed breached the corporation's editorial guidelines "by calling Donald Trump a racist".
Mr Johnson said: "I'm so sorry, I just don't, I've never heard of her."
Pressed by Ferrari again, who asked "You don't know the story? You've never heard of Naga Munchetty?", the Conservative Leader replied: "No, I'm sorry."
The radio host then said the "row over racism", to which Mr Johnson replied: "You're going have to forgive me. I should know about Naga Munchetty, but does anybody know about Naga Munchetty. Hands up? You know about Naga Munchetty?"
Lord Hall said in correspondence to BBC staff the ECU ruling had sparked "an important debate about racism" and the interpretation thereof.
He added: "Racism is racism and the BBC is not impartial on the topic. There was never a finding against Naga for what she said about the president's tweet."
After reviewing the ECU decision himself, he said: "I don't think Naga's words were sufficient to merit a partial uphold of the complaint around the comments she made.
"There was never any sanction against Naga and I hope this step makes that absolutely clear. She is an exceptional journalist and presenter and I am proud that she works for the BBC."
The BBC faced a backlash in the days after the ruling and a number of prominent black and Asian journalists and broadcasters called for the decision to be reversed.
READ MORE: Naga Munchetty: BBC reverses complaint
Ms Munchetty has not yet commented and is due to return to present BBC Breakfast on Thursday alongside Charlie Stayt.
The controversy means the corporation's impartiality has been presented as "a fault", according to a former director-general.
Mark Thompson, who was head of the BBC from 2004-12, said the row over Ms Munchetty's comments about her own experiences of racism meant the corporation had had "one of its virtues turned against it".
Speaking at the Headmasters' and Headmistresses' annual conference in London today, Mr Thompson said: "To people in a calm mind, most people would recognise why the BBC should think hard about impartiality, whatever anyone thinks about the Naga Munchetty case."
He was referring to a comment headline in Tuesday's Times newspaper which said: "Kneejerk impartiality has driven the BBC mad."
The New York Times Company president added: "In this headline, impartiality has become a fault, it's become a negative, unthinking, emotional response.
"The BBC's virtues, one of its virtues has been turned against it.
"That's a characteristic of a long-running story."
When asked explicitly whether the BBC had made the right decision by reversing the ruling, Mr Thompson did not offer an opinion.
He said: "I thank my lucky stars for many things; one thing I am thankful for right now is that is not my job anymore.
"To give you an opinion I would need to study the case much more closely than I have."
In the 17 July broadcast, Ms Munchetty said: "Every time I have been told, as a woman of colour, to go back to where I came from, that was embedded in racism."
She added: "I'm not accusing anyone of anything here, but you know what certain phrases mean."
The BBC's original ruling sparked a backlash, with many expressing their outrage at the decision.
Sir Lenny Henry and Krishnan Guru-Murthy were among a group of black and Asian journalists and broadcasters who called for the BBC to reverse its ruling.
Lord Hall said on Monday: "I don't think Naga's words were sufficient to merit a partial uphold of the complaint around the comments she made."