Saatchi, 70, said he was "utterly bereft" that a private email he sent to Lawson referring to her alleged drug use had been made public.
The email he wrote to Lawson was read in full to jurors at the trial of two Italian sisters who were personal assistants to the couple. The two are accused of fraudulently spending 685,000 pounds (USD 1.1 million) on Saatchi's company credit cards.
"If you ask me whether I actually knew whether Nigella ever took drugs, the answer is no."
Saatchi said had been "very fond" of Elisabetta and Francesca Grillo - who deny fraud - but feared they would use their allegations that Lawson took drugs as a defence in their trial for fraud.
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In the email he referred to his ex-wife as "Higella" at one point.
He wrote: "I can only laugh at your sorry depravity.
Questioned by a prosecution lawyer, Saatchi said: "I'm utterly bereft that this private email to Nigella has come back to haunt us both.
"The stories that the Grillos were parading was that Nigella had a severe cocaine habit that stretched back a very long time for the entirety of our marriage.
"What I was speculating here was that the Grillos would use this as a defence."
Saatchi said he told the pair they could pay off the money they owed and live rent-free in a London house that Lawson used for filming, but his then-wife did not agree.
Prosecutors claim the Grillo sisters spent the money on first-class flights, hundreds of taxi journeys and designer clothes and handbags.