The embassy did not disclose the reason for refusing Lawson entry into the United States.
Embassy spokeswoman Lynne Platt said Thursday that Lawson was stopped from traveling Sunday and had subsequently been invited to the embassy to apply for a visa. She said such applications were generally handled "routinely and expeditiously."
Lawson tweeted Saturday that she was going on vacation, but she also has worked in the U.S., where she co-hosted TV cooking competition "The Taste."
British citizens need visas to work in the U.S. but not for a vacation.
U.S. authorities may refuse passengers admission for various reasons, including drug use.
Lawson, author of "How To Be A Domestic Goddess," has had a turbulent year in which her personal life was scrutinized in the media.
In July she divorced art collector Charles Saatchi after he was photographed grabbing her throat outside a London restaurant.
In December she testified at the fraud trial of two former aides, and told the court she had used cocaine a handful of times. She denied claims by the defendants that she was a regular drug user.
"I promise you ... regular cocaine users do not look like this," said Lawson, who is known for her voluptuous figure.
Police later said they would not be investigating Lawson.
Lawson's co-host on "The Taste," chef Anthony Bourdain, tweeted that he was "absolutely mortified with embarrassment over the cruelty and hypocrisy of U.S. actions" over Lawson's travel.
Bourdain told The Associated Press in an email that Lawson was "the most focused, non-party, sober person I know. How this could happen - to her, of all people - is beyond me."
A spokesman for Lawson declined comment.
She is not the first British celebrity to fall afoul of U.S. immigration officials. In 2008 the late singer Any Winehouse, who had a marijuana arrest and well-publicized problems with alcohol and drugs, was refused a visa to perform at the Grammy Awards.
Singer Lily Allen was denied a visa the same year, shortly after she was arrested over an altercation with photographers.
AP food editor J.M Hirsch contributed to this report.