Lawson was barred from boarding a flight from London's Heathrow airport to Los Angeles on Sunday, British tabloid the Daily Mail reports.
The self-styled domestic goddess revealed during last year's trial of Elisabetta and Francesca Grillo that she had taken cocaine and smoked cannabis. The sisters were later acquitted.
It's believed Lawson, 54, tried to register online for permission to travel to the US and confirmed she had not been arrested or convicted of offences including taking illegal drugs.
US authorities, however, can bar foreigners who have committed such offences, even if they haven't been charged.
Lawson can apply to have the no-fly ban lifted, but it could be a lengthy and bureaucratic process.
Lawson's television career includes being a judge on the US talent show The Taste.
The chef's 10-year marriage to art collector Saatchi, 70, ended in divorce after he accepted a caution for assault when pictures emerged of him with his hand around Lawson's throat at a London restaurant in June 2013.
