The killer of Royston children's author Helen Bailey was accused of murdering his first wife at their Bassingbourn home when examination of brain tissue donated to medical research showed her "breathing had been restricted".

The murder trial involving the death of Diane Stewart began at Huntingdon Crown Court yesterday, with the prosecution opening statement being heard today.

Opening the case in front of the jury, Stuart Trimmer QC said: "The Crown say this defendant intended to kill Diane Stewart and the only serious issue you have to determine is whether Ian Stewart was responsible for the killing or whether it was a medical accident."

The court heard that Ian Stewart, who is 61 and originally from Letchworth, was initially "able to fool medical professionals by suggesting his wife, Diane Stewart, had died in the course of an epileptic fit" in 2010, prosecutor Stuart Trimmer QC said.

The barrister said that although most of Mrs Stewart's remains were cremated, "she had donated her brain to medical research and brain tissue was kept".

Police investigated the death of Stewart's first wife after a jury found him guilty in 2017 of the murder of Ms Bailey at the home she shared with Ian Stewart in Baldock Road, Royston. They discovered her body along with that of her dog Boris in a cesspit under the garage.

Scientists and pathologists were instructed to re-examine the circumstances, and analysis of the brain tissue indicated Diane Stewart's death was "most likely caused by a prolonged restriction to her breathing from an outside source".

The prosecutor told jurors: "The conclusions, in short, are these - the cause of death was most likely a prolonged restriction of her breathing from an outside source, and that the account given by Ian Stewart, the only other person on the premises, is directly contradicted by the medical evidence."

Mr Trimmer said Mrs Stewart's death, on June 25, 2010, happened on a morning when the defendant's two sons were away.

He said Stewart gave "differing accounts" of what happened, including that he had been out to Tesco and came back to find his wife outside on the ground, and that she had been hanging washing out and he found her lying next to the line.

He answered no comment in police interviews, instead providing a prepared statement which said: "On that day I had left our home and, when I returned a short while later, I found Diane lying unconscious on the patio.

"I went inside to get the cordless house phone and dialled 999."

Mr Trimmer said: "Diane Stewart's body was cremated, but her brain was kept for medical research and was, by a stroke of fortune, available for recent expert examination.

"Three pre-eminent experts have re-examined all the material and will be relied on by the Crown."

He said Mrs Stewart had a "mild form of epilepsy that was well controlled" and she had not had a seizure for 18 years.

One consultant neurologist gave her risk of Sudden Unexpected Death in Epilepsy (Sudep) at the time as "extremely low - one in 100,000", Mr Trimmer said.

The prosecutor said the "narrative involving an epileptic fit that day appears to have originated from Ian Stewart, rather than from any other source".

Mr Trimmer said that, following Mrs Stewart's death, Stewart's behaviour was "hard to square with the conduct of a grieving husband".

"He was shortly out buying a sports car and embarked upon a new relationship," the barrister said.

"Friends of Diane recall his behaviour at the funeral as being unusual."

Jurors were told that Ian Stewart formed a relationship with Royston's Helen Bailey, described in court as a "successful children's author", following the death of Diane Stewart.

Prosecutor Mr Trimmer told the jury: "In 2016 this defendant murdered Helen Bailey, killed her dog and dumped both dog and Helen Bailey in a cesspit.

"He was convicted of that murder in February 2017."

Mr Trimmer said that while investigating that "particularly callous crime", police officers and scientists began to look again at the death of Diane Stewart.

He pointed out that in each case the victim was a woman in an intimate relationship with Stewart.

Stewart was flanked by four officers as he sat in the secure dock, listening to the prosecutor open the case.

The trial, set to last up to four weeks, continues.