Alan Williams, the prosecution pathologist, knew of “clear evidence” that Harry, the second baby, had an infection but he kept the results secret, leading to a serious miscarriage of justice, Clare Montgomery, QC, said.

Ms Montgomery was opening Clark’s appeal against her convictions and life sentence for smothering 11-week-old Christopher in 1996 and for shaking Harry to death when he was eight weeks old in 1998.

Clark, a corporate lawyer, has always protested her innocence and her husband, Stephen, has led a tireless campaign to clear her name. Her first appeal failed when three judges ruled that the case against her was overwhelming. But the case has now been referred back by the Criminal Cases Review Commission, which investigates possible miscarriages of justice.

Yesterday Ms Montgomery told the court that the fluid in a healthy baby should be sterile, protected from harmful substances in the blood by the blood brain barrier. But microbiological tests on Harry Clark found that he was infected with the staphylococcus aureas infection that had spread to his cerebral spinal fluid. “In all likelihood he died suddenly in reaction to the staphylococcus aureus bacteria with which his stomach, lungs and cerebral spinal fluid were riddled.”

Ms Montgomery continued: “It requires no degree of expertise to realise that the test results should have been revealed by Dr Williams. It is obvious that, if the results had been made known at trial, Sally Clark would not have been convicted of murdering Harry or his brother, Christopher.”

In May 2001, the Solicitors Disciplinary Tribunal made the unprecedented decision not to strike Clark off the Roll of Solicitors, after hearing an account of the case and watching a personal appeal by Clark from her prison cell recorded on video.

Court eight of the Royal Courts of Justice was packed with her friends and relatives, many of them professionals who have worked on the case without fees.

Her husband was there with her father, Frank Lockyer, a retired divisional commander with Wiltshire Constabulary.

Clark, 38, was jailed for life after being found guilty of murder with a 10-2 majority verdict at Chester Crown Court in November 1999.

Her husband spent the last three years trawling through the evidence and found a microbiology report in Harry’s previously undisclosed medical notes. Those notes form the crux of the appeal. They were obtained from Macclesfield General Hospital by Mr Clark and Marilyn Stowe, a family solicitor in Cheshire who offered her help,.

Several experts also believe that Christopher, the first baby, was suffering from the same bacterial infection as Harry at the time of his death, but to a lesser extent. But as the cause of death had originally been given as natural causes, there was thought at that time to be no reason to carry out further tests that might have revealed that Christopher died the same death as his brother.

Ms Montgomery said that Dr Williams must have withheld evidence “deliberately” since the prosecution must have asked questions about whether there were any test results relating to Harry that had not been disclosed. She said that it appeared that sole responsibility for the non-disclosure lay with Dr Williams, whose explanation was “unsatisfactory”. “His claim that there was no evidence of infection in Harry’s case was, on its face, untrue.”

Ms Montgomery, in her outline case, also argued that the deliberate non-disclosure also has an impact on “Dr Williams’s credibility, competence and claimed impartiality”.

Another of the fresh grounds of appeal is expected to centre around the discovery in February 2001 of a “cot death gene” by researchers at Manchester University.

The Interleukin 10 genotype was found to be associated with a threefold increase in the risk of cot death. Clark’s defence team believe that this discovery blows out of the water evidence presented at her trial that the chance of two cot deaths in her family stood at one in 73 million.

At the 2000 appeal Lord Justice Henry, Mrs Justice Bracewell and Mr Justice Richards rejected an argument that the jury could have been misled by the statistical evidence.

The appeal continues.

Explore UK News

• Crime News

• Education News

• Health News

• Science News

• Scotland News

Times Recommends

• DIY video helps the WI get to grips with sex
• Artists urge nation to save Titians
• Go home, Iraqi leader tells Britain