Charles Saatchi has used the fraud trial of his two former assistants to attack his ex-wife Nigella Lawson because he was looking for a place "to put his hurt and anger" over their disintegrating marriage, a court heard on Wednesday.

Anthony Metzer QC, representing Elisabetta Grillo who is accused of the unauthorised spending of £105,000 on credit card and taxi accounts used by the Saatchi/Lawson household, put it to the jury that the art dealer considered Grillo "a soft underbelly" through which to attack the TV chef.

"Could it be that Mr Saatchi was using this as a way to attack Miss Lawson by turning on one of his most trusted and loved people?" Metzer asked the jury as they prepared to begin their deliberationsat the end of a case that has seen the TV chef admit to using cocaine and cannabis. "The extravagant way Miss Lawson kept her extended family on his money was now for him the legitimate place to assert his feelings."

"Miss Lawson and Mr Saatchi set out to bolster their own reputations in court at each others expense," he added. "My client has been caught in the collateral crossfire like a child in an acrimonious divorce".

Grillo, 41, denies the fraud charge she faces alongside her sister, Francesca, 36 who is accused of defrauding the household of £580,000.

Elisabetta, left, and Francesca Grillo, who are accused of fraud. Photograph: Andrew Matthews/PA

Lawson, Metzer alleged, failed to tell the truth about the extent of her drug use in a bid to save her career. "Ms Lawson would have realised that a full acceptance of her drug use would have a significant effect upon her credibility and her career as an admired public figure," he said.

He told the jury Lawson's evidence of limited drug taking "should be approached with extreme caution", saying it "came from a woman caught between a rock and a hard place" and that she had been "bullied" to maintain that she had not authorised the Grillos' spending.

She did this "in fear of an investigation being carried out into her lifestyle which would have disclosed her authorisation of generous personal expenditure by her staff on his credit card as well as her secret drug use", Metzer said.

He told the jury "this is a case with no winners – not Mr Saatchi, not Ms Lawson, and certainly not my client".

Closing for the prosecution, Jane Carpenter told the jury the Grillos had been responsible for a four year "greedy and fraudulent free for all … abusing the trust and generosity of their employers."

"The defendants could have simply said they were authorised to use the cards," said Carpenter. "Why didn't they. Not once did they say that. Do you think it wasn't said because it wasn't true?"

Carpenter earlier put it to Francesca Grillo that she lied about what they knew of Lawson's cocaine use. She denied the claim.

In her closing submission on behalf of Francesca Grillo, Karina Arden urged the jury to put aside Lawson's celebrity status when considering the drug allegations against her.

"I ask you to look at her not because she is a famous TV cook, but look at her evidence in exactly the same way you would anybody, dare I say it anybody who lives on an estate," she said.