Prime minister David Cameron has been rebuked by a judge for speaking of his admiration for cook Nigella Lawson while she is involved in a court case.

Judge Robin Johnson told jurors hearing the case against two of the celebrity chef's personal assistants accused of defrauding her that they felt "aggrieved" by Cameron's comments.

"It is of regret when people in public office comment about a person who is involved in a trial which is in progress," the judge said, telling the jurors to ignore them.

"It is inconceivable that some of your number may not have seen these comments.

"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 that they may feel aggrieved is not without justification."

"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."

Cameron had told the Spectator magazine that he was a "massive fan" of the cook who he found to be "very funny and warm".

When asked if he was on "Team Nigella", the prime minister replied: "I am."

The prime minister's spokesman said he would not be commenting on the affair.

Sisters Francesca and Elisabetta Grillo deny fraud and abusing their positions by spending £685,000 on credit cards belonging to Lawson and husband Charles Saatchi.

They claim they were given permission to use the business cards by Lawson to cover for her use of drugs, including cocaine.

Lawson has admitted in court to using drugs a handful of times, but denied she had a drug problem or was an addict.