Video: Did Helen Bailey murderer kill his first wife, too?

The 56-year-old smothered the sedated Electra Brown writer and dragged her body into a cesspit as part of a plot to acquire her riches.

She was found in the foetid burial site beneath their garage three months after she abruptly disappeared in April last year.

Dead at her side was her pet dachshund Boris.

Stewart was convicted of the “despicable” killing in a unanimous verdict at St Albans Crown Court, prompting police to turn their focus to the death of his first spouse.

Diane Stewart, an epileptic who was mother to his two sons Jamie and Oliver, was found dead in the couple’s garden in June 2010. An inquest at the time concluded she died of natural causes.

Detective Chief Inspector Jerome Kent, who helped secure Stewart’s conviction, told the Press Association: “You will not be surprised that police investigating Ian Stewart for the murder of Helen Bailey would consider if there are any similar links to the death of his first wife.

“There is not a murder investigation into Diane Stewart, there is a re-examination of a sudden, unexpected death. It is only right that I would look back on somebody’s past.”

The former software engineer, wearing a light blue shirt and jeans, showed no emotion as he was found guilty by a jury of seven men and five women of murder, fraud, preventing a lawful burial and three counts of perverting the course of justice.

As he was led from the dock, he fixed his gaze on Jamie, his eldest son, who did not meet his eye.

Outside the courtroom, tearful relatives and friends of 51-year-old Ms Bailey embraced.

Her family welcomed the verdict, reached after five and a half hours of deliberation, but said her death had left them in a “long shadow of loss”.

Stewart targeted the vulnerable widow on the internet in 2011, earning his way into her trust and later her £3.3 million estate after launching a “love-bombing” offensive.

“She was being grossly deceived by someone who was preying on her,” prosecutor Stuart Trimmer told the trial.

Over many weeks, Stewart surreptitiously fed Ms Bailey his prescription anti-insomnia drug, Zopiclone, possibly by slipping it into her morning scrambled eggs.

She soon became panicked by her deteriorating state of mind, searching online for terms such as “can’t stop falling asleep” and expressing concern to loved ones.

A pillowcase found next to her body led the prosecution to suggest Stewart used a pillow to smother her while she was stupefied by the sedatives.

Ms Bailey, known for her young adult stories and memoir on bereavement, was found submerged in the tank of human sewage on July 15.

Her brother John sat in court almost every day of the trial, while their elderly mother Eileen broke down as she gave her evidence.

The family said in a statement: “Despite this victory for justice there can be no celebration. Our families have been devastated and nothing can ever bring Helen back to us, or truly right this wrong.

“A long shadow of loss has been cast over the lives of so many who will always remember Helen with enduring love and affection.”

The couple met on a Facebook group for the bereaved and started a relationship within a year of Ms Bailey’s first husband drowning on holiday.

Stewart soon became the chief heir to her fortune in a rewritten will and gained power of attorney over her affairs.

He stood to gain around £1.8 million from her investment portfolio, plus the value of their home in Royston, Hertfordshire, and her coastal cottage in Kent.

Hours after the murder, he illicitly boosted a standing order to himself from her account.

This earned him an extra £12,000 over the three months following her disappearance, during which he played the part of a man wounded by his bride-to-be’s abandonment.

A financial inquiry will be held to ensure Stewart does not profit from his crime, prosecutor Charles White said.

He added: “He was an arch dissembler, he was able to trick everyone, so I think anybody who came across his path was a potential victim.”

Stewart sparked a major search after telling Ms Bailey’s loved ones she had left abruptly to seek some “space” at her seaside cottage in Broadstairs.

Detective Chief Inspector Jerome Kent said: “To kill somebody was despicable enough, but to dispose of her in the way he did and lie to everyone including his own children shows how wicked and despicable that man is.”

Footage of a half-dressed Stewart being arrested during an early-morning swoop on his home on July 11 was shown to the court, in which he said to officers: “You’re joking.”

His deception culminated with a fantastical tale of a deadly kidnap plot by two mystery men called Nick and Joe who, he claimed, coerced him into lying for eight months about Ms Bailey’s fate.

Stewart will be sentenced by Judge Andrew Bright at 10am on Thursday.

At her home in Chelmsford, Mrs Stewart’s sister Wendy Bellamy-Lee said she had no comment to make on the conviction.

Responding to the fresh police inquiry, she added: “I will follow the case as it happens.”