Marianne Williams, 24, was said to have known that Joshua Taylor’s underdeveloped kidneys could not process the salt before giving him the equivalent of 3½ teaspoonsful because she felt she could not cope. But a jury at Winchester Crown Court took only 2½ hours to clear her of both murder and manslaughter.

Ms Williams said afterwards: “I’m just overwhelmed. It has been a long time coming. I keep having floods of relief going through me. I can’t concentrate or even think about what has happened.”

But in an attack on the “devastating” decision to press charges, her solicitor Jacqui Cameron said: “We, as lawyers, are very concerned that the parents — usually mothers — are continuing to face the grave charge of murder on what can only be described, at best, as complex and, at worst, dubious evidence. Clinicians are far too prone to point fingers of suspicion at persons who are already vulnerable.

“The verdict gives us hope that the general public is learning the lessons that doctors are failing to learn, despite other high-level cases such as Sally Clark, Trupti Patel and Angela Cannings.”

This was the second case involving salt to come before the courts this year. In April a couple were freed 15 months after being jailed for the manslaughter of a boy they were hoping to adopt. Ian and Angela Gay were said to have force-fed Christian Blewitt, 3, with salt. Their convictions were quashed by the Court of Appeal, but they remain on bail awaiting a retrial.

Ms Williams had also been accused of force-feeding her son excessive amounts of sodium. He was taken to Southampton Hospital in February 2004, but died three days later.

The court was told that Ms Williams had seen the consequences of too much sodium in his system earlier that month when he had been taken to hospital. But the prosecution’s experts, a consultant responsible for the child’s day-to-day care and a national paediatric authority, were unable to show any “smoking gun” evidence against Ms Williams.

The defence said that Joshua died as a result of a combination of his medical condition and his medication.

Joshua was born 12 weeks prematurely by emergency Caesarean section on November 26, 2002, weighing just 2lb 10oz (1.2kg). His twin had died in the womb. Joshua had a number of medical problems, including very small kidneys, and the trial was told that because they were failing, they would expel too much sodium. He was therefore given daily doses of sodium chloride that were necessary for health and growth.

After the boy’s earlier admission to hospital, doctors led by the consultant Rodney Gilbert gave him drugs containing salt to treat him for dehydration. During the trial, Dr Gilbert admitted using an equation for adults in calculating what drugs to use to bring down sodium levels in the body. He said that he now understood the equation was not designed for children. He also admitted that glucose and urine tests were not carried out that could have given a clearer indication of Joshua’s salt levels.

It was Dr Gilbert who raised the suspicion that Joshua may have been poisoned deliberately, and police raided Ms Williams’s home the same day.

Professor George Haycock, an expert witness for the prosecution, admitted during the trial that it was impossible to say whether Joshua’s high salt levels had occurred before he was taken to hospital or while he was being treated there.

Detectives bugged the family home but gained no evidence from the covert operation, the trial was told.

Detective Inspector Matt Johnson said yesterday: “The investigation was complex and required sensitivity, thoroughness and detailed liaison with medical experts and other agencies, including social services. I acknowledge the verdict reached by the jury and would add only that I am satisfied that every possible piece of evidence that could be found was properly presented to the jury.”

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