It was claimed that TV chef Nigella used cocaine, cannabis and prescription pills on a daily basis and let the Italian duo use Charles' company credit card to keep her secret.
The 53-year-old 'Domestic Goddess' had the 'non-verbal' agreement with sisters Francesca and Elisabetta Grillo in return for an understanding that they wouldn't leak details of her drug use, the court was told.
"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," Anthony Metzer QC, representing 32-year-old Elisabetta, told the court.
Nigella is expected to give evidence during the trial which is due to last at least two weeks.
It is alleged that Nigella used the drugs for more than a decade but kept her habit a "guilty secret" because she knew her husband "would not condone" her behaviour.
Art collector Charles sent his ex-wife an email saying he believed "every word" of the sisters' claims and added that he thought she had authorised their spending because she was "off her head" on drugs, Isleworth Crown Court in London heard on Tuesday.
The claims of Nigella's drug were exposed after the long-term assistants were charged with cheating PR magnate Charles out of more than £300,000 to spend on holidays, designer handbags and other items.
The sisters, who worked with Nigella for more than 10 years, wanted to use the "understanding" over Nigella's alleged drug use as part of their defence.
A judge ruled that the "bad character" claims could be used in court meaning the millionaire chef can be cross-examined about them when she gives evidence in the case.
Between 1 January 2008 and 31 December 2012 the Grillo sisters allegedly committed fraud by using the credit card for personal spending. They were arrested in August 2012 after Charles became aware of their spending. Elisabetta, 41, and Francesca, 35, both of Kensington Gardens Square, Bayswater, west London, both deny the charges.
The duo were first employed by Nigella when she was married to her first husband journalist John Diamond, who died of throat cancer in 2001.
Francesca and Elisabetta were described by Nigella as her "kitchen confidantes" in a dedication in a cookbook published in 2011.
Earlier this year she filed for divorce from her husband of 10 years on the grounds of his "continued unreasonable behaviour".
Their relationship came to an end after pictures were published of Charles holding Nigella's throat at a top London restaurant.
Charles initially dismissed the incident as a "playful tiff" but later accepted a police caution for assault. He told the Mail on Sunday that the pictures gave a "wholly different and incorrect implication".
Nigella and Charles were estimated to be worth £150million together and previously shared an £11.8m home in Chelsea, London.