The cunning computer engineer – who was determined to get his hands on Ms Bailey’s £4million fortune – will now almost certainly die behind bars.
And at the end of his seven-week trial police announced it would be “only right and proper” to look again at the death of his wife Diane, who suffered an epileptic seizure seven years ago.
Police are to re-examine death of Ian Stewart's first wife Diane
A jury had been told how Stewart plied Ms Bailey, 52, with sleeping pills for months before smothering and dumping her in a foul cesspit.
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
Detective Chief Inspector Jerome Kent
It is possible she was still clinging to life when he shoved her in the hole under the garage of their £1.5million home in Royston, Herts.
Stewart even killed Ms Bailey’s beloved dachshund Boris and threw him in beside her along with his favourite toys.
After today's guilty verdict at St Albans Crown Court the officer who led the police investigation, Detective Chief Inspector Jerome Kent, said: “To kill somebody was despicable enough.
Police are to probe the death of Ian Stewart's first wife Diane
“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.”
Grieving relatives of Ms Bailey 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.”
Earlier the killer’s own QC, Simon Russell Flint, had told the court: “He acknowledges that he had burdened the family of both Helen Bailey and his own family with incalculable loss, a lifetime of misery, sorrow and suffering, and he will have a long time to reflect upon the consequences of those actions.
“There is every prospect and likelihood that Mr Stewart will end his days behind bars.”
When Ms Bailey vanished in April last year, Stewart – who claimed he loved and wanted to marry her – told family, friends and police she had left and taken Boris because she wanted “space and time alone”.
He was reluctant to give police a statement but was eventually videoed by officers as he spoke to them about her “going missing”.
Stewart was reluctant to give police a statement but was eventually videoed by officers
He had modelled their descriptions on two innocent acquaintances. When the men were brought into court Stewart lamely told the jury they “don’t compare at all” with the non-existent kidnappers.
The jurors saw through all the grey-haired father of two’s lies and unanimously found him guilty.
He was also convicted of fraud, perverting the course of justice and preventing a lawful burial.
Stewart will appear tomorrow to be sentenced by Judge Andrew Bright. As he was remanded in custody to await his fate, police confirmed they are to re-examine his wife’s death.
Det Chief Inspector Kent added: “Diane Stewart died of natural causes in 2010. It would only be right and proper that we re-looked at what the causes might be.
“There is no suggestion she died of anything other than natural causes but it will be part of our inquiries.”
Stewart gave a statement claiming Helen had been kidnapped by two mystery thugs called Nick and Joe
Ms Bailey, best known for stories featuring teenagers Electra Brown and Daisy Davenport, met Stewart in 2012 via an internet bereavement support group.
She was deeply vulnerable after her husband of 22 years, businessman John Sinfield, had drowned on a holiday in Barbados. Stewart “love-bombed” her with affection and she soon became besotted.
To Ms Bailey, Stewart was a “GGHW – gorgeous, grey-haired widower” she wanted to marry.
To Stewart, she was a ticket to a fortune. And as she planned their wedding, he was plotting her murder.
For months he secretly plied her with increasing doses of sleeping pills called Zopiclone, leading her to complain of feeling constantly tired.
For months Stewart secretly plied her with increasing doses of sleeping pills
In a chilling twist, Ms Bailey’s brother John was once touring Hartwell Lodge and she herself joked to him the cesspit was a “good place to hide a body”.
On the day Stewart killed her and Boris, he changed a monthly standing order from her account to his from £600 to £4,000.
In the weeks to come he sent a string of texts to her phone, paid for posters and flyers appealing for information about her and went on a dog walk organised by a group called “Where is Helen Bailey?”
He paid to renew their Arsenal FC season tickets and even went on holiday to Spain – but his charade never fooled detectives.