Breitbart News is a leading backer of U.S. president-elect Donald Trump; its chief executive Stephen Bannon has been appointed as one of Mr Trump's key White House aides. He has been accused of promoting divisive white nationalism.
The school said the decision to cancel the talk was taken "following contact from the DfE counter-extremism unit, the threat of demonstrations at the school by organised groups and members of the public, and our overall concerns for the security of the school site and the safety of our community".
More than 200 6th-formers had signed up to attend the event with parental consent, the school said, adding that "objection to our hosting Mr Yiannopoulos came almost entirely from people with no direct connection to The Langton; the staff and students of the school were overwhelmingly in favor."
The DfE said it contacted the school after concerns were raised by members of the public.
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"When concerns are raised by members of the public following media coverage in advance of an event, the department would contact the school as a matter of routine to check they had considered any potential issues," she said. "The decision to cancel the event was a matter for the school."
Mr Yiannopoulos, who is gay and was banned from Twitter earlier this year in a racism row, responded to the cancellation of his talk by posting on his Facebook page: "Who even knew the DoE [sic] had a counter-extremism unit? And that it wasn't set up to combat terrorism but rather to punish gays with the wrong opinions? Perhaps if I'd called the speech 'Muslims are awesome!' they'd have left us alone. Disgusted."
He told the Sun newspaper that the education department's intervention indicated that prime minister Theresa May was "a total fascist".