Expert witnesses have lost their 400-year-old immunity from being sued in the civil courts.
Today's historic decision by the Supreme Court follows a complaint by motorcyclist Paul Wynne Jones who was injured after he was hit by a car in Liverpool in March 2001.
His solicitors instructed consultant clinical psychologist Sue Kaney to prepare a report in support of his damages claim against the driver who was drunk, uninsured and driving while disqualified.
She concluded that Mr Wynne Jones was suffering from post-traumatic stress disorder but the defence consultant psychiatrist said he was exaggerating his symptoms.
The two experts were ordered to prepare a joint statement, and it was Mr Wynne Jones's case that Ms Kaney carried out this task negligently in signing a report agreeing that he had not suffered PTSD and had been deceitful.
This was so damaging to his claim that he settled for a significantly lower sum than he might have otherwise been awarded, and led to him issuing proceedings for negligence.
A High Court judge struck out Mr Wynne Jones's case, as he was bound by a 2000 Court of Appeal decision that an expert witness was entitled to immunity, and the matter went to a panel of seven Supreme Court justices as a point of general public importance.
Giving their reasons for allowing the appeal, by a majority of five to two, Lord Phillips rejected argument that expert witnesses would be discouraged from providing their services if they were liable to be sued for breach of duty.
"All who provide professional services which involve a duty of care are at risk of being sued for breach of that duty. They customarily insure against that risk."
He added that a lesson was to be learnt from the position of barristers.
"It was always believed that it was necessary that barristers should be immune from suit in order to ensure that they were not inhibited from performing their duty to the court.
"Yet removal of their immunity has not in my experience resulted in any diminution of the advocate's readiness to perform that duty.
"It would be quite wrong to perpetuate the immunity of expert witnesses out of mere conjecture that they will be reluctant to perform their duty to the court if they are not immune from suit for breach of duty."
Concluding that the immunity from being sued for breach of duty should be abolished, whether in contract or in negligence, he emphasised that this did not extend to the absolute privilege enjoyed by experts in respect of defamation claims.
Dissenting with Lord Hope, Lady Hale said the topic was more suitable for consideration by the Law Commission and reform, if thought appropriate, should be considered by Parliament rather than the Supreme Court.