A judge's decision to allow expert witnesses immunity from disciplinary proceedings over remarks made in court had no support from the discredited paediatrician Sir Roy Meadow or from the General Medical Council, the Attorney General told the Court of Appeal yesterday.

As guardian of the public interest, Lord Goldsmith was making a rare intervention in the GMC's appeal against a High Court ruling by Mr Justice Collins in February.

Sir Roy, whose statistical error in 1999 led to the wrongful conviction of Sally Clark for the murder of her two young sons, had appealed against a finding by the GMC's fitness to practise panel that he was guilty of serious professional misconduct and should be struck off.

Allowing his appeal, Mr Justice Collins created a new immunity for all experts covering evidence they gave in court. The only exception would be if a judge had referred the case to a disciplinary body.

Before the court began hearing the GMC's appeal yesterday, Lord Goldsmith told three senior judges, headed by the Master of the Rolls, Sir Anthony Clarke, that the immunity would cut down a disciplinary body's freedom to investigate complaints.

"This would be an illegitimate development for the common law," he said. "It is crucially important that expert witnesses should assist the court conscientiously and objectively, rather than being tempted to give any evidence that suits their client's case."

He said it was inappropriate for the judge to lay down new law when there was no urgent need to do so; it was inconsistent because experts would not be freed of the fear that they might face proceedings if a court referred the case up; and it was unprincipled because someone else's trial was not the place to assess an expert's competence.

There would also be practical problems, for example in cases in which an expert's incompetence did not come to light until after a case was over.

Lawyers for the GMC argue that the decision over Sir Roy rendered it "toothless" against professionals acting as experts in court cases.

The court is expected to reserve its judgment.