Mr Justice Royce accepted that Brian Blackwell, 19, who took his girlfriend on a £30,000 international spending spree after the killings, had acute narcissistic personality disorder. He ordered that a double murder charge be dropped, allowing Blackwell to admit manslaughter on the grounds of diminished responsibility.

Five psychologists agreed that Blackwell exhibited the classic symptoms of the condition, including a “grandiose” self-importance, a desperate desire for admiration, an undeserved sense of entitlement and, crucially, a tendency to fly into a rage when thwarted.

Blackwell was a brilliant student who achieved four As at A level and planned to study medicine. But in the grip of his delusions he believed he was on the verge of becoming a world-ranked tennis star.

Liverpool Crown Court was told that in July last year he attacked his parents, Jacqueline, 60, and Brian, 70, at their bungalow in Melling, Merseyside, with a claw hammer and a kitchen knife. They had tried to stop him going on a trip to Miami with his girlfriend, Amal Saba, 18, to celebrate his “breakthrough” in the tennis world. Although he had 9p in the bank, Blackwell wrote Miss Saba a cheque for £39,000 as her first three-months salary as his full-time personal secretary.

David Steer, QC, for the prosecution, said: “The defendant is a highly abnormal young man. In killing his parents he has committed a grossly abnormal act, apparently in a frenzied and raging attack.

“The Crown are unable to resist the persuasive logic that this severe abnormality of mind has substantially impaired his responsibility at the time of the killings.”

Blackwell, a pale figure in an open-neck beige shirt, sobbed uncontrollably in the dock as a letter was read to the court in which he expressed his bitter regret for his crimes.

“Every moment of every day I wish I could turn back the hands of time,” he had written. “I eternally long to be a little boy again at a time when everyone really loved each other, when we could have a happy time and be a family once more. I miss them more than anything in the world.”

Mr Justice Royce sentencing Blackwell to life imprisonment, told him: “The circumstances in which you bludgeoned and stabbed to death first your father and then your mother are chilling. You then, with breathtaking callousness, left their bodies to rot while you enjoyed a luxury holiday in America with your girlfriend. For a son to do this is almost beyond belief but you are no ordinary son.”

As Blackwell is suffering from a personality disorder rather than a recognised mental illness, he will not immediately be sent to a psychiatric hospital. He is expected to start his sentence in a young offender institution, where he will be assessed by psychologists before being transferred to either Frankland or Whitemoor top security jails.

Blackwell, known as “Brains” to schoolfriends and “Little Brian” at home, was brought up as a cosseted only child. He became an “exemplary” scholarship student at the £2,335-a-term Liverpool College, in Mossley Hill.

In February last year he found his first serious girlfriend in Miss Saba, the daughter of Jordanian-born physicians. She wept in the public gallery as Mr Steer told the court how Blackwell had boasted to her that he had a £70,000 tennis sponsorship deal from Nike. He showered her with jewellery, test-drove a Mercedes with her and stole £9,000 from an account opened by his parents to save for his university fees to buy her a car.

His parents were becoming alarmed, Mr Steer said. They were also getting in the way of his vision. Nevertheless, Blackwell asked his girlfriend to join him on a trip to Miami. He spent £12,500 on his father’s credit card on flights to and between New York, Miami, Barbados and San Francisco.

On July 25 last year a violent row erupted at the family home, which ended with Mr and Mrs Blackwell’s deaths.

The following day Blackwell and Miss Saba flew to New York before going to Florida, Barbados, and San Francisco.

On their return, Blackwell moved in with Miss Saba, saying that his parents were in Majorca. During the six weeks before he was detained he made 13 credit card applications and ran up bills of £30,000.

On Sunday, September 5, police discovered the bodies of Mr and Mrs Blackwell. When arrested Blackwell asked the officers: “Is prison cold?”

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