Nigella Lawson, the TV chef and self-styled “domestic goddess”, had a chronic drug habit and secretly used cocaine, cannabis and prescription pills “daily” for more than a decade, a court has heard.

As the very public disintegration of her marriage to the tycoon Charles Saatchi returned to the spotlight, the allegations were made by lawyers defending two Italian sisters, Francesca and Elisabetta Grillo, who worked as assistants to the couple in their London family home.

The pair are accused of defrauding Ms Lawson and Mr Saatchi – who divorced in July, ending a 10-year marriage – of more more than £300,000 while working as their assistants.

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But according to the assistants’ testimony, Ms Lawson had a verbal understanding with the Grillos that they could use Mr Saatchi’s company credit card for their personal use in return for keeping quiet about her drug use.

The drug claims first emerged in a pre-court trial hearing on the 15 November when Anthony Metzer, representing Elisabetta Grillo, took the unusual step of lodging a “bad character application” in order to question Ms Lawson’s reliability as a witness for the prosecution. Mr Metzer told Isleworth Crown Court in west London the application “relates to Miss Lawson’s alleged taking of class A and class B drugs and her unauthorised use of prescription drugs” and that it was kept as a “guilty secret” from her husband.

“She did not want him to know about her use, particularly of cocaine. Because the defendants were fully aware of her illicit drug use, she consented to their expenditure on the understanding there would be no disclosure to her husband of her drug usage,” he said.

Jane Carpenter, for the prosecution, said: “This is a totally scurrilous account which has been raised by the defence, and the timing is no coincidence at all.” She said that despite being arrested more than a year ago and charged in March, the sisters only made the drug allegations to the court earlier this month.

The claims were made public yesterday, however, when Judge Robin Johnson decided they were admissible in court.

The judge read out an email sent from Mr Saatchi to his former wife in which he described her as "Higella" and claimed Ms Lawson was “so off [her] head on drugs” that she failed to monitor their spending patterns.

“Of course now the Grillos will get off on the basis that you [and] Mimi [her daughter] were so off your heads on drugs that you allowed the sisters to spend whatever they liked and yes I believe every word they have said,” the email read.

The judge told the court that Mr Saatchi said: “At the time of sending the email I was completely astounded by the scale of drug use set out in the statements [from the defendants]. Nevertheless I did believe the allegations that I’m referring to in the email.”

Judge Johnson said the allegations by Mr Saatchi were some of the many communications since the “unhappy events surrounding the witnesses’ divorce this summer”. The pair broke up after pictures were published in a newspaper in June showing the art dealer holding his wife by the throat. The incident, was dismissed by Mr Saatchi as nothing more than “a playful tiff” but he later accepted a police caution for assault.

The trial starts on Wednesday.