London (CNN) -- Charles Saatchi, the millionaire ex-husband of celebrity chef Nigella Lawson, said Friday he believed his ex-wife had taken drugs but had no evidence of it, as he testified at the trial of two former assistants accused of fraud.
The former assistants, Italian sisters Francesca and Elisabetta Grillo, deny embezzling hundreds of thousands of pounds on company credit cards.
Allegations of drug use by Lawson were made in a pre-trial hearing for the case earlier this week, when an e-mail sent by Saatchi to Lawson in October was read out by the defense.
In the e-mail, Saatchi wrote that the assistants would likely "get off" because Lawson was using cocaine and marijuana on a daily basis and "allowed the sisters to spend whatever they liked."
Giving evidence Friday in the west London court, Saatchi said he was "bereft" that the e-mail -- in which he said he "believed" what the sisters claimed about Lawson's drug use -- had been made public.
He said he never saw his wife taking drugs during their 10-year marriage and that he had no hard evidence she had done so.
But he testified that the accusation "sounded pretty compelling to me."
The allegations emerged in June, around the time that the couple were photographed in a restaurant having an argument.
In the photos, which were splashed across the front pages of national newspapers, Saatchi is seen with his hand around Lawson's throat. Saatchi accepted a police caution for assault, and the couple announced they would divorce soon afterward.
The prosecution also showed Saatchi a list of the top 50 transactions made by the sisters, totaling more than $1 million, which he said he did not recognize and had not authorized.
He was not able to say whether Lawson had approved them, he said.
Earlier, Saatchi described the sisters' roles as housekeeping, organizing the household and helping out with any tasks needed by the family. They were first employed by Lawson and then by his company, he said.
Saatchi said he was aware the two sisters were given credit cards in order to pay for household expenses. This was his ex-wife's idea, he told the court.
Asked about the sisters' role when Lawson's children grew older, Saatchi said: "I'm very fond of Francesca and Lisa (Elisabetta) and they were part of our family ... The children were fond of them, we didn't want them to go, so they stayed with us as housekeepers and for general assistance."
The first witness in the case, Rahul Gajjar, an accountant for Saatchi, told the court Thursday that Francesca had 64,336.97 pounds ($104,500) in expenses on her company credit card in June 2012.
Among the expenses were purchases from designer labels Prada, Miu Miu, Louis Vuitton and Chloe, as well as hotel bills and plane tickets.
The court heard that Francesca's average monthly expenditures were 48,000 pounds and Elisabetta's were 28,000 pounds, whereas other personal assistants to Saatchi, a well-known art collector, spent a maximum of 8,000 pounds a month.
The sisters were expected to make purchases for the household on the company credit card, Gajjar said.
But the prosecution says they spent large sums of money on themselves.
Drug use claims
Isleworth Crown Court heard at a pretrial hearing for the sisters this week that Lawson and Saatchi had a "culture of secrecy" around their marriage.
Representatives for Lawson declined to comment on the drug allegations then, citing the ongoing court proceedings.
The defense alleged that the e-mails showed both Saatchi and Lawson had attempted a "manipulation of the court" and that the case should be thrown out. The judge ruled the trial would proceed.
The prosecution told the court Wednesday that the two sisters spent about 685,000 pounds ($1.1 million) on company credit cards over the course of four years, according to UK media reports.
Lawson, whose culinary skills and flirtatious manner have long entranced UK audiences, has also appeared as a judge on ABC show "The Taste" in the United States. A second season of the show is due to air in January.