A couple jailed for the "salt overdose" killing of a three-year-old boy they planned to adopt protested their innocence yesterday after their convictions were quashed but they were told they must face a retrial.

The Court of Appeal held that, if the jury at the original trial of Ian and Angela Gay had heard fresh evidence suggesting that toddler Christian Blewitt could have died of natural causes as a result of a type of "salt diabetes", the verdict might have been different.

The Gays, who had served 15 months of their five-year jail terms for manslaughter, were released on bail pending a retrial later this year.

Ian Gay (39) and his 40-year-old wife, from Halesowen, were jailed in January last year following a seven-week trial at Worcester Crown Court.

Christian died in hospital four days after being found unconscious in his room on December 8 2002 at a £500,000 house where the wealthy childless couple then lived in Bromsgrove, Worcestershire.

Former engineer Mr Gay and his wife, a £200,000-a-year insurance actuary, always insisted they loved Christian and his younger brother and sister, who had been placed with them for a trial period with a view to adoption.

Defence lawyers argued in the appeal that the jurors were presented with just two options - either the couple murdered Christian by blunt force to the head or were guilty of manslaughter through feeding him salt as a punishment for naughtiness.

According to a new witness, Dr Glyn Walters, the boy was not a victim of salt poisoning and could have been suffering from a rare condition which allowed sodium levels to build up in the body to the point of overload.

The condition could explain why Christian, alleged to have been force-fed up to six teaspoons of salt, equivalent to a litre of sea water, had so much sodium in his system.

Lord Justice Richards, sitting in London with Mr Justice Penry-Davey and Judge Ann Goddard, said they had not lost sight of the fact that the medical evidence, although important, formed only part of the evidence in the case.

The couple, having recently taken on the "immense burden" of looking after three children with a view to adoption, were "in a position of considerable stress".

Difficulties they had experienced with Christian had added to that stress.

But this did not alter the possibility that the fresh evidence could have influenced the outcome of the trial.

The convictions were "unsafe", the judge said.

Arguing against a retrial, defence counsel Michael Mansfield QC said that, in such a unique and complex case, the scientific dilemma facing the medical experts could not be solved by asking a fresh jury to "grapple" with it.

"There is a very high risk of a further conviction that might be regarded as unsafe because it is a case based in the realms of speculation," he said.

But the judges ruled that a retrial should be ordered "in the interests of justice".

In a statement outside the High Court, Mr and Mrs Gay said: "The convictions against us have been quashed. However, we must now face the fresh agony of a re-trial.

"We now know for certain what we knew all along - that Christian died of natural causes."

The couple thanked their legal team and added: "We thank all of our family and friends for all their love and support.

"We would also like to thank the media for their support over the last 12 months.

"We are now just looking forward to going home.

"Yet again we protest our innocence and hope that one day soon true justice will finally be done."

Sylvia Heal, MP for Halesowen, said she was pleased for the couple.

"I am certainly pleased that they have had their conviction quashed.

"I'm sure their family is extremely pleased, they have spoken of their innocence throughout the case.

"There has been other cases where people have been convicted and then subsequently alternative medical evidence has been found. I hope this is the case for the Gays," she said.