Spectator columnist Rod Liddle has become the first blogger to be censured by the Press Complaints Commission.
On the Spectator's website, Mr Liddle wrote that the "overwhelming majority" of London's violent crime was carried out by young, African-Caribbean men.
But the PCC ruled the former BBC Radio 4 Today editor's words breached Clause 1 (Accuracy) of its code.
It said the "significant ruling" showed publications' websites would be held to the same standards as print editions.
A reader had complained after the article was published in December 2009.
Mr Liddle had written that "the overwhelming majority of street crime, knife crime, gun crime, robbery and crimes of sexual violence in London is carried out by young men from the African-Caribbean community".
PCC director Stephen Abell said the Spectator had provided some evidence to back up Mr Liddle's assertion.
But he added "it had not been able to demonstrate that the 'overwhelming majority' of crime in all the stated categories had been carried out by members of the African-Caribbean community".
Mr Abell said the ruling was significant because it demonstrated that "the PCC expects the same standards in newspaper and magazine blogs that it would expect in comment pieces that appear in print editions".
"There is plenty of room for robust opinions, views and commentary, but statements of fact must still be substantiated if and when they are disputed," he said.
"And if substantiation isn't possible, there should be proper correction by the newspaper or magazine in question."
Mr Abell said that although several readers had taken issue with Mr Liddle's claim in the comments section of the blog, the PCC did not agree that "the magazine could rely on publishing critical reaction as a way of abrogating its responsibilities under the code".
Following the ruling, Mr Liddle said the PCC had "got it wrong".
He said: "It seems to suggest that had I not used the word overwhelming, they wouldn't have come down against me.
"I'll confess and maybe I shouldn't have used the word 'overwhelming' in all the cases I quoted.
"But a blog is different because it has to be a conversation, otherwise there's no point in having a blog."