Judge Robin Johnson said Cameron's remarks, made in an interview with a journalist, were "favourable" to Lawson.
Elisabetta and Francesca Grillo, who used to work for Lawson and ex-husband Charles Saatchi, are accused of fraudulently spending £685,000 on designer clothes and handbags, first-class flights and taxi journeys.
The judge, sitting at London's Isleworth Crown Court, told jurors he had seen press reports of Cameron's comments about Lawson.
"It is of regret when people in public office comment about a person who is involved in a trial which is in progress," he said.
"The defendants feel aggrieved as the comments, although they do not specifically deal with matters in the trial, are favourable to Ms Lawson.
"The fact they they may feel aggrieved is not without justification."
He added: "You will realise that what public figures may feel about this case or a witness in this case can have no bearing on the issues that you have to decide."
During the trial, Lawson has accused millionaire art collector Saatchi of "peddling" stories about her alleged drug habit.
These included that he was checking her nose for cocaine when he was photographed gripping her throat outside a restaurant in London's Mayfair this year.
She did admit taking cocaine on a handful of occasions, including with her ex-husband John Diamond shortly before he died of cancer in 2001.
Elisabetta, 41, and Francesca Grillo, 35, of Bayswater, west London, deny committing fraud by using a company credit card for personal gain between January 1 2008 and December 31 last year.