A woman has been sentenced to nine years in prison after confessing to killing her father and burying his body in her back garden in Greater Manchester after a “lifetime of abuse” at his hands.

Barbara Coombes, 63, walked into a police station in Stockport on 7 January this year and told officers that she had killed her father 12 years earlier.

Detectives began a murder investigation and started digging up her garden in Reddish, Stockport. Two days after she confessed, police found the body of her father, Kenneth Coombes, a second world war veteran. He would have been 87 at the time of his death.

She later claimed he had sexually abused her for more than 40 years, since she was five, and had used her like a “sex slave”. He raped her hundreds of times throughout her life, she told psychiatrists.

He may even have been the father of her first child, David, who died shortly after birth, her barrister, Martin Heslop QC, told Manchester crown court on Wednesday.

When Coombes was aged somewhere between six and nine, her father took her to a photography club where he forced her to display her genitals while other men took photographs of her, she claimed.

The abuse continued right until he died, she said. He would constantly touch her breasts even as she entered her fifth decade. She said she had no friends, no hobbies, had never worked and only rarely left Reddish her whole life.

Coombes told police that she “snapped” one day in January 2006 after discovering naked baby photographs of her herself and another child among her father’s belongings. She feared she was not her father’s only victim, she said, “and a black cloud appeared over me”.

She said she grabbed a spade she had been using in the garden, went into the living room and hit her father on the back of the head. He asked her what she was doing and she said that she feared he would “inflict life-threatening injuries on me or kill me” and used the sharp end of the spade to cut his throat. She then wrapped his body in an old carpet and hid him from her daughter, Islay, who was then 18, and buried him the following day.

At Manchester crown court on Wednesday, Coombes pleaded not guilty to murdering her father but guilty to the lesser charge of manslaughter by reason of diminished responsibility.

Sentencing her to nine years, the judge Timothy King said he did not accept she acted in self-defence. However, he accepted she killed while suffering from post-traumatic stress disorder and severe depression as a result of “40 years of extreme mental, physical and sexual abuse at the hands of your father”.

Her rational judgment was impaired and she was unable to exercise self-control, he told her, noting that she had attempted suicide on a number of times in her youth and had self-harmed throughout her life.

The judge said he did not believe Coombes would ever have confessed had “the net not started to close in around her”. A representative of Stockport housing association had become suspicious of Kenneth Coombes’s whereabouts and was due to make a house visit the day after his daughter confessed.

The housing officer made several attempts to check on his welfare but was repeatedly sent away by Coombes, who on one occasion claimed her father – who then would have been 99 – was at a Buddhist retreat in Manchester.

For the past 12 years she had been fraudulently claiming benefits amounting to £189,125 – both his pension and carers’ allowance for herself. Her barrister, Martin Heslop QC, told the court she was caught in a “catch-22”, unable to stop claiming the money because she could not admit to anyone that her father was dead.

She never reported him missing. Neighbours said they assumed he had moved away. Coombes told Islay that he had died suddenly from blood poisoning and had been cremated.

In a victim impact statement, Islay Coombes said her heart was broken at what had happened and how she had been deceived but would stand by her mother. “I hope when this is done we can repair our relationship to something approaching normal,” she said.

Michelle Colborne QC, prosecuting, said the crown accepted the plea and could not contest defence claims that Coombes had suffered “a lifetime of abuse – verbal, physical and potentially sexual, at the hands of the deceased”.

But she questioned whether Coombes was truly remorseful for what she had done. The police officer who met her at Cheadle Heath police station when she confessed was struck by how she showed “little or no emotion”, Colborne said.

The barrister noted that Coombes only began to talk of being sexually abused several months after her arrest, during her fourth interview with a psychiatrist.

In April this year Coombes appeared in court and denied murder but admitted preventing her father’s lawful burial. She also pleaded guilty to fraud and false representation.

After the sentencing, Duncan Thorpe, the senior investigating officer in the case, said Coombes “showed absolutely no concern for what she had done and denied everyone the chance to say goodbye, as Kenneth lay buried at the bottom of his own garden, just metres from her own bedroom window.

“Despite having years to tell someone what really happened, she only came forward when she had no other choice.”