Salt poison couple's court appeal

But the couple were convicted of his manslaughter in January last year.

Lawyers for the couple will tell the Court of Appeal the boy suffered from a rare condition which accounts for the high salt levels found in his body.

William Bache, from the couple's new legal team, told BBC Breakfast the condition was "fairly obscure and many people simply don't know about it, including the doctors".

Mrs Gay's brother, Carl Swain, said he hoped the argument would clear his sister and her husband.

"It's a very rare condition and the first time round at trial, obviously it was missed and the jury weren't presented with any other cause other than salt poisoning.

"We've got a very well respected expert who we hope will explain it to the three judges, and they will hopefully see that there is a natural explanation to how Christian died."

The Gays, who are both 38 and moved to Halesowen in the West Midlands after Christian's death, were jailed for five years each following their convictions.

Christian was taken to Russells Hall Hospital in Dudley on 8 December 2002, after being found unconscious in his room. He died four days later.

Tests found he had excessive levels of sodium in his blood and CT scans established he had suffered brain damage.

Medical evidence was inconclusive, and after the trial one expert witness for the defence, Home Office pathologist Dr Peter Acland, suggested there was "significant doubt" about the guilty verdict.