Boris Johnson said seeing a "bunch of black kids" used to make him "turn a hair" and run away in a little-known column opposing anti-racism reforms after Stephen Lawrence's murder.
The Tory leader was branded "fundamentally racist" by a veteran QC over the 19-year-old remarks - which admitted he "prejudged" that black youths in the park could mug him.
In the column he said the "tiny fit of prejudice" was "disgraceful" and "disgusting", but claimed it was "as natural as sewage".
The then-journalist wrote: "If that is racial prejudice, then I am guilty. And so are you, baby. So are we all."
The column, which was buried deep in a 2000 edition of The Guardian, has been largely forgotten since Mr Johnson entered 10 Downing Street.
That is despite other writings from the same era being well-known - including branding gay men "tank-topped bum boys" and black people "piccanninies".
He claimed Macpherson's reforms were "just as wrong" as Enoch Powell, the MP infamous for his racist 'rivers of blood' speech.
And he claimed a key reform to let victims and third parties define if something is racist - now the national standard for prosecutors - was "Orwellian stuff" from the "PC brigade".
He suggested we "axe large chunks of the anti-racism industry" and instead have a "reasonable legal framework for minimising the problem like the infrastructure used to remove sewage”.
Mr Mansfield said: “This is a man who is deeply prejudiced and obviously I’m horrified about the possibility that he may remain Prime Minister. He is fundamentally sexist and racist.”
He added: "It seems to me that this wasn’t an isolated moment of prejudice - either his observations about a 'bunch of black kids', or his observations about Macpherson."
Mr Mansfield said he was “not surprised at all” with the “public school banter” from an “upper class” Prime Minister.
Labour candidate David Lammy added: "Stephen Lawrence’s horrific murder and the institutional racism evidenced by the Macpherson report showed beyond doubt the desperate need for anti-racism reforms.
“The fact Boris Johnson used his well-paid, privileged platform to oppose those reforms and to normalise prejudice shows beyond doubt that he is unfit to be our Prime Minister.
“Johnson was on the wrong side of history then, as he is now when he engages in dog-whistle racism and panders to the far right."
In the 2000 column, Mr Johnson wrote that he had Turkish heritage but added: "I am guilty none the less. Not of racism, I hope, but of spasms of incorrectitude, soon over, soon regretted.
"When I shamble round the park in my running gear late at night, and I come across that bunch of black kids, shrieking in the spooky corner by the disused gents, I would love to pretend that I don't turn a hair.
"Somehow or other a little beeper goes off in my brain. I'm not sure what triggers it... but I put on a pathetic turn of speed."
Mr Johnson said "maybe" he would also run if it was a "gang of white kids" but "I'm not sure. I cannot rule out that I have suffered from a tiny fit of prejudice.
"I have prejudged this group on the basis of press reports, possibly in right-wing newspapers, about the greater likelihood of being mugged by young black males than by any other group.
"And if that is racial prejudice, then I am guilty."
He added: "And so are you, baby. So are we all. If there is anyone reading this who has never experienced the same disgraceful reflex, then well, I just don't believe you."
The column ran a year after High Court judge Sir William Macpherson published his report into the racist murder of Stephen Lawrence at a bus stop in 1993.
A key recommendation was to define racist incidents as "any incident which is perceived to be racist by the victim or any other person".
Prosecutors now identify racist incidents are "any incident which is perceived by the victim or any other person to be motivated by hostility or prejudice based on a person's race."
But writing in 2000, Mr Johnson said that could lead to a "crazy" situation with people criminalised for references taken out of context.
He wrote: "Where the left, the Guardian, Macpherson and the whole PC brigade are just as wrong as Powell, is in thinking that we should endlessly hunt for evidence of one of humanity's worst features, tease it out, legislate for it, bang on about it, create thousands of jobs financially dependent on discovering it."
A spokesman for Boris Johnson declined to comment.