Angela Cannings was jailed for life in April 2002

Angela Cannings, 40, from Salisbury in Wiltshire, was convicted last year of smothering seven-week-old Jason in 1991 and 18-week-old Matthew in 1999.

She has always maintained they were victims of sudden infant death syndrome (SIDS).

And on Tuesday an expert witness told her appeal hearing the boys had been at a "substantially increased risk" of dying from cot death.

'Greater risk'

Medical statistics expert Professor Robert Carpenter said Cannings' sons had been laid to sleep on their front, and may have been exposed to cigarette smoke, both factors which gave them a higher chance of SIDS.

The professor, a medical statistics expert with 50 years' experience in the field, said one of the boys was bottle fed for the first two weeks of his life, which could have caused a reaction that put him at even greater risk.

"The Cannings family smoked and the children slept prone... the children were at substantially increased risk," he said.

"Matthew was bottle fed and proteins may have got into the blood causing an anti-genic response and potentially resulting in a hyper-sensitive state."

'Unexplained events'

Professor Carpenter, who has advised a governmental inquiry into SIDS, said these factors might explain three deaths in one family - Matthew, Jason and Cannings' first child, Gemma.

In the weeks before their deaths, both boys had suffered unexplained events which, it was alleged, were caused by an attempt to smother them.

But Professor Carpenter said that such an assault would be likely to leave marks on the child, such as bleeding around the eyes and nose, none of which were found on the Cannings children before or after death.

It was more likely that the children were hyper-sensitive due to an allergy, genetic defect, or a response to risk factors such as tobacco smoke, he said.