Children's author Helen Bailey was gradually drugged and "probably suffocated" by her partner before being dumped in a cesspit below her home, a court has heard.

The 51-year-old writer was found deep in the grounds of her sprawling £1.5million property in Royston, Hertfordshire, in July 2016, three months after she was reported missing.

Near her side lay the remains of her beloved miniature dachshund, Boris.

Ms Bailey's fiance, Ian Stewart, 56, of Baldock Road, Royston, is accused of killing her after he "secretly administered" a sleeping drug in increasing amounts over time.

Opening his trial at St Albans Crown Court, prosecutor Stuart Trimmer QC said: "The crown say this was simply a long-planned, deliberate killing, a cynically executed murder that had money as its driving motive."

Jurors heard that the killing allegedly took place on April 11 at the home the couple shared, before Ms Bailey's body was hidden in a septic tank below the garage.

Her death put the defendant in line for "substantial financial advantage", the court was told.

Stewart is charged with murder, three counts of perverting the course of justice, preventing a lawful burial and fraud. He denies all the accusations.

On the day the author was allegedly killed, her bank account was accessed, with a standing order to the defendant bolstered from £600 to £4,000, the court heard.

Mr Trimmer said: "(The couple) had made financial arrangements that meant, in the event of the death of Helen Bailey before the wedding, Ian Stewart would obtain the house and a very substantial financial advantage."

At the time of her killing, Ms Bailey was said to be worth more than £4million.

The victim was said to have been concerned in the weeks leading up to her death that she was feeling unnaturally sleepy.

"The crown case is that this defendant secretly administered to her a sleeping drug in increasing amounts," Mr Trimmer said.

He added: "All the typical activity that Helen Bailey engaged in ceased at about 11am Monday, April 11 2016.

"The crown case is that, by early afternoon, the only other person in that house, Ian Stewart, killed her, probably by suffocation whilst she was sedated by the drugs he had administered for some time."

After Ms Bailey's death, her dog was also killed to give credence to the theory that she had gone missing because she was "devoted" to him, Mr Trimmer told the court.

After calling the police, Stewart is alleged to have sent texts to her phone, despite having possession of it himself, and threw his weight behind a national press campaign to trace his missing partner.

CCTV was also said to have captured him visiting a recycling centre in Royston and disposing of a "large white object", alleged to be a duvet used at some stage in the killing or subsequent cover-up.

Following this, the jury heard, he "casually obstructed" the police investigation, lying to police officers and parking a car over the access point to the waste tank where her body lay.

He also went on holiday to Spain in June 2016, jurors were told.

Mr Trimmer added: "Between April 11 and July 15, the crown say the defendant sat back and watched the police conduct what he knew to be a futile missing person investigation."

When Ms Bailey's remains were eventually discovered, traces of a sleeping drug prescribed to Stewart for insomnia were found in her body, the court heard.

After Stewart was arrested, he told police that two men - named Joe and Nick - were responsible for the killing and the disposal of the body.

Ms Bailey, who previously lived in north London, was known by younger readers for her characters Electra Brown and Daisy Davenport, but found a new audience with her blog, Planet Grief, about becoming a widow.

The trial continues.