London (CNN) -- A former personal assistant to Nigella Lawson told a court Friday that the celebrity chef had lied in her testimony as a prosecution witness.
Elisabetta Grillo, who is accused alongside her sister Francesca of embezzling hundreds of thousands of pounds from Lawson and her ex-husband, Charles Saatchi, insisted under cross-examination she had told the truth in her own defense. Both sisters deny fraud.
On Thursday, Elisabetta Grillo testified that over the years she had worked for the family, she had seen signs of cocaine use by Lawson "every three days, I don't know, regularly."
In her testimony last week, Lawson confirmed she had taken cocaine half a dozen times and used cannabis in the past. But she denied being a habitual user, saying, "I did not have a drug problem, I had a life problem."
Asked by the prosecution Friday if she thought Lawson had lied, Grillo replied, "Yes." She gave the same response when asked if Saatchi had lied.
Saatchi had said in an e-mail that Lawson had used drugs regularly, but in his testimony before the court he backed off that claim.
In the e-mail, which the defense shared with the court in a pretrial hearing, Saatchi wrote that the two assistants would probably "get off" because Lawson was using cocaine and marijuana on a daily basis and "allowed the sisters to spend whatever they liked."
Grillo said she had never seen Lawson take drugs but said she was aware that she used cocaine and cannabis. She recounted finding a packet containing white powder in the bathroom, rolled up bank notes, and credit cards and CDs with traces of white powder on them.
Asked by her defense attorney how she felt about saying Lawson took drugs, Grillo said she didn't like it. But she said it was true.
The two sisters are accused of spending large sums on luxury goods on company credit cards they were supposed to use for household expenses.
Questioned by the prosecution Friday, Elisabetta Grillo insisted that Lawson had authorized spending on personal items, such as clothes, designer shoes and cosmetics, on her work credit card.
She also said she'd been told she could take out cash on the card.
Grillo described Lawson and Saatchi as generous, saying the latter had paid for her to go to Paris in 2008 for her birthday.
The trial has been closely followed by the UK media because of its high-profile witnesses, Lawson and Saatchi, a millionaire art collector.
Allegations of drug use by Lawson emerged in June, around the time she and Saatchi were photographed having an argument in a restaurant.
In the photos, which were splashed across the front pages of national newspapers, Saatchi is seen with his hand around Lawson's throat and appears to pinch and look up her nose. Saatchi accepted a police caution for assault, and the couple announced they would divorce soon afterward.
Lawson, widely known in Britain for her cookbooks and TV shows, has also appeared as a judge on ABC's "The Taste" in the United States.
The trial continues next week.