Donald Jr commented: 'You've got to be kidding me?!'
It comes as an attacker killed three people near the Houses of Parliament in what authorities are treating as a "terrorist incident". The vehicle attack began when the purported terrorist ran through the gates of the Palace of Westminster with a vehicle before exiting and stabbing a police officer.
Trump's son, Donald Trump Jr., manages the Trump Organization and has no role in the White House, but he remains an occasional surrogate for his father.
Donald Trump Jr slammed Sadiq Khan after Wednesday's attack in Westminster over comments the Mayor made in an interview previous year about terrorism.
The article was originally posted in September 2016, and was a plea from the Mayor for Londoners to be vigilant to the dangers of living in a big city.
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"Part and parcel of living in a great global city is you've got to be prepared for these things, you've got to be vigilant, you've got to support the police doing an incredibly hard job", said Mr Khan, who was in NY at the time.
Speaking on his LBC show, Mr Farage blasted: "I don't want these things ever to be part and parcel of living in a modern, major city. That means being vigilant, having a police force that is in touch with communities, it means the security services being ready, but it also means exchanging ideas and best practice", Khan told the Evening Standard.
It seems that Trump Jr shared the article without reading the piece at all.
"Nothing is more important to me than keeping Londoners safe", Khan added.