Nigella Lawson took cocaine to help her work through the night when she was writing her cookery books, a jury has been told.
Ms Lawson found it easier to work during the night and "needed help" to stay awake, it was claimed.
The class A drug caused a change in her personality, leaving her "absent and grumpy", according to her former housekeeper Francesca Grillo.
Ms Grillo and her sister Elisabetta are on trial at Isleworth Crown Court in London accused of fraudulently spending £685,000 (€809,838) on corporate credit cards issued by Ms Lawson's ex-husband Charles Saatchi. They deny the charges, saying Ms Lawson authorised their spending.
Nigella Lawson told the court during her own evidence that she used cocaine only once during her 10-year marriage to Mr Saatchi, but Ms Grillo said it was much more widespread.
She said: "At the beginning of 2012, there were a few episodes where she came down and she had white powder inside her nostril. A few times I ignored it but other times I told her and she said 'it's just make-up'. It was too white to be make-up.
"When she was writing a new book, she said she needed to work through the night because she found it easier and she needed help to stay up all night. She had a runny nose for a long period of time, and at a strange time, in the summer."
Ms Grillo said she would find rolled-up banknotes with white powder on them in Ms Lawson's handbags when she was transferring the contents of one bag into another for her.
Asked what effect the drugs had on Ms Lawson, she said: "She was going from being very kind and nice to being a bit absent and grumpy.
"She had always been a caring, warm person but in the last few years she had started to be grumpy, moody, she had trouble sleeping."
Sometimes when she had trouble sleeping, Ms Lawson would come downstairs and smoke cannabis with her children because "it helped her sleep", the defendant said.
Ms Grillo (35) said that when she saw pictures of Ms Lawson's bust-up with Mr Saatchi in Scott's restaurant in Mayfair in June, she decided the couple must have been arguing about drugs.
"The picture which stuck in my mind was Charles picking her nose. I thought maybe he had the same problem I had, he found some remains inside her nose relating to drugs and I thought maybe if he didn't know that maybe he didn't know that she authorised all the spending.
Ms Lawson would often ask her to find items she had mislaid, including keys, her phone and a tube of English mustard she always carries.
She said that when she was accused of fraud last year Charles Saatchi summoned her to his office and accused her of buying a house with his money.
"He was banging on the table and saying I would end up in handcuffs," she said. "I told him I had not hidden anything, I had been here all the time, and he said, 'You can f*****g hide anywhere in Italy but I'll find you'.
"He said he was going to destroy me and hunt me down, that was his words. You don't cross Charles Saatchi, everyone knows that."
Recalling spending time in a prison cell after she was arrested last year, Ms Grillo said Mr Saatchi had pursued a "personal vendetta" against her, adding: "I couldn't believe that the people I cherished and loved had put me in that position. I still don't believe it."
The trial continues. (© Daily Telegraph, London)