Saatchi said he would sue Lawson for £300,000 at centre of Grillo fraud trial if she did not give evidence, court hears

Charles Saatchi threatened to sue his ex-wife Nigella Lawson if she refused to give evidence in a trial in which their former assistants are accused of defrauding the couple out of more than £300,000, a court has heard.

Saatchi's lawyers wrote a pre-action letter to the TV chef in October after she indicated she was reconsidering whether to give evidence in the trial of Elisabetta Grillo, 41, and her sister Francesca, 35.

Saatchi said he had heard allegations that his former wife had a drug habit which she carried out in the family home, which he believed to be true. The court heard the Grillos, who sat impassively in the dock at Isleworth crown court, had become caught in "a huge internal wrangle" between the chef and her former husband. They are accused of "grossly abusing their position of trust" as they worked for the couple throughout their 10-year marriage, which ended in June.

On Tuesday the court heard claims that Lawson had a daily drug habit, taking cocaine, class B and prescription drugs over a period of at least 10 years. It was said that she allowed the Grillos to use the couples' credit cards and accounts in exchange for their not letting her husband know about her drug use.

The pre-action letter, partly read out in court, said: "The claimants [including Saatchi] are also concerned that Ms Lawson gave the defendants' [the Grillos] permission to use the accounts for personal purposes whilst under the influence of drugs and/or that Ms Lawson has no credible recollection of events as a result of drug abuse."

The threatened civil action demanding Lawson repay the money allegedly taken by the Grillos emerged as lawyers for the sisters argued there had been an abuse of process in the case and it should be dropped because the former couple, who are also the main prosecution witnesses, are "at loggerheads".

"There has been a manipulation of the court process by the two main prosecution witnesses in this case," said Anthony Metzer QC representing Elisabetta Grillo. "It is a convenient forum for Mr Saatchi and Miss Lawson to reprise a disputed issue between them in relation to their marriage in the criminal courts when the subject of libel, short of anything malicious being alleged, would not be possible."

He added: "They are so much at loggerheads, this is not a situation of having a different recollection. Mr Saatchi is saying Miss Lawson is a criminal because she has been taking class A drugs in her home.

"The defence case is that Nigella Lawson lied to her ex-husband about her drug use and about the expenditure both expressly and implicitly because she was fearful about Mr Saatchi's reaction if he knew about the culture of drug use and the expenditure."

The judge, Robin Johnson, did not accept the defence's application for the trial to be stopped as a result of an abuse of process and allowed the case to continue.

The court heard how Saatchi came to learn of his ex-wife's alleged drug use.

In a statement taken from the art collector on Monday, he said: "During our marriage I was unaware that Nigella took drugs. It is only during the breakup that I became aware she had been taking drugs whilst we were married – although I was not aware of the scale of the problem. I believe it was in about June 2013, at the time of the well publicised incident at Scott's restaurant – around the same time I was told about her drug use by my daughter Phoebe. It was confirmed by other people in and around the household."

Lawson is expected to be cross-examined in the trial as one of the prosecution's key witnesses. Saatchi is also expected to be called into the witness box as the crown seeks to prove the fraud case against the Grillos.

The case continues.