Lawyers for Clark, jailed for life at Chester crown court in 1999, claimed on the first day of her appeal that such a "material non-disclosure" of documents constituted a "serious miscarriage of justice".

Clark, 38, was convicted of smothering 11-week-old Christopher in 1996 and shaking eight-week-old Harry to death in 1998.

But her counsel, Clare Montgomery, QC, said the investigation and trial had been skewed by the decision "deliberately" to withhold key test results. In a written statement, Ms Montgomery said: "For nearly three years after the death of Harry Clark, Sally Clark, her lawyers and their medical advisers believed there was not any evidence of infection or any possible natural explanation of his death."

The court heard that at the end of 2000 it emerged for the first time that there had been clear evidence of an infection with staphylococcus aureus that had spread as far as the baby's cerebral spinal fluid.

The evidence had apparently been known to prosecution pathologist Alan Williams since February 1998, Ms Montgomery said. "It is now clearly appreciated by those who are specialist in the area that this provides a very real explanation as to how Harry met his death. It requires no degree of expertise to realise the test results should have been revealed by Dr Williams."

Robin Spencer, QC, for the crown, said test results were not deliberately withheld.

The hearing continues today.