Sir Roy Meadow, whose evidence helped to send three innocent mothers to jail for killing their babies, today successfully appealed against being struck off the medical register.
Mr Justice Collins ruled that the General Medical Council's punishment against Sir Roy, 72, for giving erroneous evidence at the trial of Sally Clark was "unduly harsh".
He warned that such disciplinary treatment could cause other doctors to become reluctant to appear in child abuse cases, citing an exodus of willing experts since the ruling. He said that Sir Roy had made a simple, honest mistake.
Mr Collins told the High Court: "He made one mistake, which was to misunderstand and misinterpret the statistics. It was a mistake, as the panel accepted, that was easily and widely made.
"It may be proper to have criticised him for not disclosing his lack of expertise, but that does not justify a finding of serious professional misconduct."
The GMC has been granted leave to appeal to the House of Lords.
In a statement, Professor Meadow - who is out of the country - spoke of his personal relief and said that the ruling had wider implications.
He said: "Children can only be protected from abuse if those who suspect abuse are able to give their honest opinion without fear of retribution. This is an important decision for paediatricians and all doctors, nurses, teachers and other professionals who may have to express difficult and sometimes unpopular opinions in the course of giving evidence in court.
"They should be able to do so without the fear of prosecution by the General Medical Council or other professional regulators."
Dr Christine Tomkins, deputy chief executive of the Medical Defence Union which has supported Professor Meadow throughout the case, said: "In 120 years, seldom have we come across a judgment as unfair and disproportionate as the GMC’s erasure of Professor Meadow from the Medical Register.
"Doctors should be able to provide expert opinion in cases where there are allegations of child abuse without fear that they will be the subject of a finding of serious professional misconduct for expressing a genuinely held belief."
A spokeswoman for the Royal College of Paediatrics and Child Health (RCPCH) said: "Professor Meadow’s work has undoubtedly saved the lives of many children. We hope that today’s decision will give confidence back to paediatricians, whose responsibility and role is so important in the protection of children."
The ruling comes two days after Lord Goldsmith, the Attorney General, introduced new guidelines to restrict the evidence given to courts by expert witnesses, after revealing that three more parents may have been wrongly jailed in the past decade.
Professor Meadow, based in Leeds, was a witness in the trials of Miss Clark, Angela Cannings and Donna Anthony, who were each convicted of murdering two of their babies but later cleared on appeal. He also gave evidence against Trupti Patel, who was found not guilty of killing her three babies.
The retired paediatrician who, over decades, has convinced courts that at least 81 infant deaths were murders, controversially told jurors that the chances of two cot deaths in a family were effectively one in 73 million - famously likening this to the probability of an 80-1 horse winning four consecutive Grand Nationals.
Mrs Anthony was jailed for life in 1998 for killing her daughter Jordan, aged 11 months, in 1996 and four-month-old son Michael in 1997. She was freed on a second appeal in April 2005 when an Appeal Court judge said evidence given by Prof Meadow was "flawed".
Mrs Anthony told Sky News today: "He ruined my life and it seems to me he's now able to ruin other people's lives. All I ever wanted was a simple apology and for him to admit he got it wrong. He has never spoken to me."
Professor Meadow's testimony that also helped to convict Mrs Clark of murdering her two infant sons Christopher and Harry in 1999 - a verdict that was quashed four years later.
Mrs Clark's family subsequently lodged a formal complaint with the GMC'S Fitness to Practice Panel. This ruled that he had abused his position as a doctor by giving misleading evidence and had strayed outside the area of his expertise. He was found guilty of serious professional misconduct.
Professor Meadow appealed and at a London High Court hearing in January, Nicola Davies, representing the professor, argued that the decision to erase his name from the medical register was unfair.
Ms Davies said Professor Meadow provided evidence in good faith and at all times genuinely believed it was correct and accurate. She said that at no time during the trial did Professor Meadow hold himself out as possessing statistical expertise.
Mr Collins said that he based today's judgment partially on concerns from the legal establishment: paediatricians have since become unwilling to give evidence which could lead to their loss of livelihood.
He said: "The decision of the FPP in this case has concerned medical practitioners who
are asked to prepare reports for or to give evidence in court. Those concerns are the more acute since the FPP specifically found that the appellant had not intended to mislead the court and that there was no evidence of any calculated or wilful failure to use his best endeavours to provide evidence. He had acted in good faith."
The judge said that there was "no doubt" that the action taken against Professor Meadow had had a damaging effect on medical experts, and warned that it may also have made experts in other fields reluctant to take the witness stand.
He said: "There can be no doubt that the administration of justice has been seriously damaged by the decision of the FPP in this case and the damage will continue unless it is made clear that such proceedings need not be feared by the expert witness."
The judge said there could not be a blanket immunity from prosecution of witnesses but added: "What is of fundamental importance is that a witness can be assured that if he gives his evidence honestly and in good faith, he will not be involved in any proceedings brought against him seeking to penalise him."
In a statement, the GMC said it was concerned that the judge appeared to have made expert witnesses untouchable.
"We agree with Mr Justice Collins that it cannot be in the public interest if doctors and other professionals are inhibited from giving evidence honestly and truthfully," the GMC said.
"However, where there has been serious judicial criticism we have sought to act to protect the public interest from experts who fall significantly short of accepted standards. In today’s judgment Mr Justice Collins has extended the principle of immunity so that it would apply for the first time to the bringing of disciplinary proceedings by regulators.
"The effect is to grant an immunity that, hitherto, has not been recognised in law. Such immunity would place doctors, and other professionals, beyond the reach of their regulator when writing reports for the courts or giving evidence. This would be true even when they, or their evidence, have been criticised by the court."
