Lawson said her ex-husband Charles Saatchi, a millionaire art collector, spread drug rumors about her after he was photographed gripping her throat outside Scott's restaurant in London. The widely published image ignited harsh criticism of Saatchi and was soon followed by their divorce in July.
"He told everyone that he was taking cocaine out of my nose at Scott's when he knows that is a lie," Lawson said, testifying at the fraud trial of two former assistants. She said her ex-husband spread "false allegations that I was a habitual user" who snorted cocaine daily.
"People who do that are a lot thinner than I am," she said. "I have never been a drug addict."
Lawson, 53, was appearing as a prosecution witness at the trial of Elisabetta and Francesca Grillo, longtime employees who worked as nannies, cleaners and assistants in the couple's London home.
The Grillos - sisters from Calabria in southern Italy - are accused of using credit cards loaned to them by Lawson and Saatchi for household expenses to instead spend 685,000 pounds (more than $1 million) on luxury clothes, accessories and accommodations at high-end hotels.
The two employees deny the fraud charges. Their defense lawyers have suggested that Lawson ignored the Grillos' lavish expenditures in return for their silence about her alleged drug use.
The media coverage of the trial has focused more on the couple's failed marriage and their tempestuous home life than on allegations that two women working for them were living the high life at the couple's unknowing expense.
Lawson, famous for her sensuous TV manner and "domestic goddess" image, described the 70-year-old Saatchi, a former advertising magnate, as a "brilliant but brutal" man with a sharp temper.
Lawson said she had been reluctant to testify in court because she had already been subjected to a campaign of "bullying and abuse" from Saatchi.
"I have been put on trial here ... and in the world's press," Lawson said.
Lawson said she had taken cocaine half a dozen times with her first husband, John Diamond, while he was dying of cancer. And she took it once again in July 2010 at a time when "I felt subjected to acts of intimate terrorism" by Saatchi.
"A friend of mine offered me some cocaine and I took it," she said. "It completely spooked me."
She also said she had smoked marijuana during the final stages of her marriage to Saatchi.
Giving her version of the notorious restaurant incident for the first time, Lawson said Saatchi grabbed her throat after she saw a baby nearby and remarked that she was looking forward to becoming a grandmother.
She told the court that Saatchi said: "I am the only person you should be concerned with. I am the only person who should be giving you pleasure."
When the pictures emerged and she fled their home, Lawson said Saatchi demanded that she come back to him and clear his name or "he would destroy me."
Lawson referred to ex-husband as Mr. Saatchi throughout her testimony. When a lawyer referred to the "unfortunate" divorce, she remarked that she didn't consider it unfortunate.
Lawson told a jury at Isleworth Crown Court in London that 41-year-old Elisabetta Grillo - known to the family as Lisa - had been "a rock" who helped her overcome the trauma of the 2001 death of her first husband.
"I loved Lisa. My children loved Lisa," said Lawson, who said she had showered Grillo with gifts, including a 7,000-pound set of false teeth. But she said her employee's behavior became increasingly bitter and unkind to Lawson's two children.
Accusations flew throughout the day in the court. Lawson used drugs and Saatchi disapproved, said Elisabetta Grillo's lawyer, Anthony Metzer. Lawson countered that Saatchi approved of some drugs, including wine and nicotine.
Metzer said his client had been "drawn in" to a battle between Saatchi and Lawson. But Lawson said Grillo had been a valued employee and friend who later had betrayed the family's trust.
Metzer read out loud a list of expenditures on the credit cards, including nights at luxury hotels and purchases from Yves St Laurent, Dolce & Gabbana and Chanel. Lawson denied authorizing them.
"It's very difficult when you find out that someone you have loved and trusted could behave in that way," Lawson said, her voice breaking.
Lawson appeared composed but tense as she was questioned about her marriage, sparring in often testy exchanges with Metzer, the defense lawyer.
"I don't see why my marriage is pertinent to you," she snapped.
But despite the alleged betrayal and the divorce stress, Lawson said she retained a positive outlook.
"I refuse to become a bitter, untrusting person," she said.