Nigella Lawson's former husband Charles Saatchi has labelled the celebrity chef a habitual cocaine user who used the drug along with cannabis and prescription pills on a daily basis during their 10-year marriage.

The billionaire businessman claimed in court that Lawson fought hard to conceal her addiction from him because she knew he would "not have condoned" her behaviour.

Saatchi made the claims ahead of a fraud trial of two sisters who used to work as personal assistants to the celebrity couple.

Sisters Francesca Grillo, 41, and Elisabetta Grillo, 35, of Bayswater, west London, are accused of defrauding Saatchi out of more than £300,000 by using Saatchi's company credit card to make extravagant purchases such as holidays and designer handbags.

The defendants deny the charges.

The court heard that Lawson was "so off her head on drugs" that she allowed the assistants to rack up a £300,000 credit card bill.

Judge Robin Johnson read out an email from Saatchi to his former wife in which he accused her and her daughter Cosima, 19, of taking drugs.

The email read: "Of course, now the Grillos will get off on the basis that you [and] Mimi 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."

Elisabetta Grillo's lawyer, Anthony Metzer, told the court: "In a nutshell we submit that she had a guilty secret from her husband - her then husband. She did not want him to know of her use of cocaine and that is highly relevant to the defence case.

"The defendants will maintain that they were fully aware of her illicit drug use and she essentially would consent to their expenditure on the understanding that there would be no disclosure to her husband of her usage."

The understanding between the sisters and Miss Lawson was not "verbalised" and was tacit, Metzer said.

"There would be an understanding between them that she would be fairly liberal in relation to their substantial expenditure, largely for themselves in addition to expenditure on behalf of the children, in return for a tacit understanding."

Lawson's lawyers described the drug allegations as "totally scurrilous" and untrue. The TV cook is expected to give evidence during the trial, which is due to last at least two weeks.

Saatchi and Lawson's marriage ended acrimoniously earlier this year after images emerged of Saatchi with his hands around her neck outside a London restaurant.

The former advertising mogul claimed that pictures showed nothing more than a "playful tiff".