The celebrity chef, who recently went through a very public and messy divorce to Charles Saatchi, was stopped at the boarding gate as she went to take her flight from Heathrow to LA on Sunday.
Nigella is currently a judge on the US TV show The Taste.
Whilst under oath during a trial last year, the 54-year-old beauty admitted that she had snorted cocaine and smoked cannabis.
According to eyewitnesses at the scene, Nigella was informed at the gate that she would not be allowed board the aircraft, and had to go back to check in to collect her bags that she had planned to stow in the hold.
US Authorities can bar visitors who have committed drugs offences - even if they haven't been charged.
GALLERY: Cocaine, Confessions and Charles Saatchi: Nigella Lawson vs Trinny Woodall
This latest humiliation comes after months of difficulty for Lawson, whose marriage to art collector Charles Saatchi broke down in June.
The pair both had to give evidence at the trial of their former housekeepers, who were accused of stealing hundreds of thousands of pounds from them.
It was during this trial that Nigella admitted she had taken cocaine on occasion, firstly to deal with the death of her first husband, and then to deal with the difficulties of living with her second.
No legal action was taken against her by British authorities for her drug admission.