Elazabeth spalding murders
Elizabeth Edwards, 49, and her 13-year-old daughter, Katie, were found dead in Spalding, Lincolnshire Family handout

Two teenagers who stabbed a mother and daughter as they slept in their beds when they were just 14 have been sentenced to life in prison.

The boy and girl, now aged 15, murdered dinner lady Elizabeth Edwards, 49, and her 13-year-old daughter, Katie Edwards, in Spalding, Lincolnshire in April 2015 in what police described as "senseless, and unprovoked attacks".

The killers were not suffering from any mental illnesses at the time of the double murder but had a "toxic" Bonnie-and Clyde style relationship, according to a psychiatric expert.

The pair, who cannot be named for legal reasons, are believed to be the youngest couple ever to be convicted of murder in Britain.

The boy admitted murder on the first day of their trial, but his former girlfriend pleaded not guilty on the grounds of diminished responsibility, as a result of abnormality of mental function.

However, the jury at Nottingham Crown Court rejected her claims and found her guilty of murder after just two-and-a-half hours of deliberation in October.

The pair have now been sentenced at the same court to life with a minimum of 20 years.

During the trial, the jury heard how the teens planned the murder of the mother and daughter and carried it out "to the letter". During police interviews following her arrest, the girl said the plan to kill the Edwards at their home started out as a joke.

Police were also alarmed at how open the girl was to confessing to her crimes, describing the act of double murder as "a breeze". After killing them both by stabbing them in the throat, the young couple went home to take a bath and watch four Twilight films together.

During the trial, consultant forensic psychiatrist Dr Philip Joseph, compared the pair to the infamous crime duo Bonnie Parker and Clyde Barrow. He said: "If they hadn't got together and had the intense, toxic relationship, they [the killings] would never have happened.

"She [female defendant] was not suffering from an adjustment disorder or a depressive illness at the time of the killings. This is clearly a planned, controlled killing – there is control all the way through it."

The female defendant told police how her then boyfriend stabbed the mother through the voice box so her daughter, Katie, would not be woken by her screams. She told detectives she did not watch the subsequent murder of the 13-year-old herself, but heard "moaning" as the teenage boy smothered her with a pillow. A post-mortem examination revealed that Katie also died as a result of stab wounds to the neck.

Spalding murder
The knife which was used in the double murder Lincolnshire Police

Investigating Officer DCI Martin Holvey said:"This has been a rare and unprecedented case and everybody who has listened to the details as they have emerged throughout the trial will, I am sure, have felt the same sense of shock and disbelief.

"The planning that went into the brutal murders of Elizabeth and Katie as they slept in their beds was cold, ruthless and chilling, as was the lack of remorse shown by the two juveniles afterwards.

"This matter-of-factness is all the more shocking given the fact that they were both only 14 years of age at the time. I hope after today's sentence, everyone affected by this terrible crime can begin to rebuild their lives and the community of Spalding can move forward."

Karen Thompson, Deputy Chief Crown Prosecutor at CPS East Midlands, added: "This is one of the most distressing and disturbing cases that I have ever encountered.

"The CPS has from the outset been confident that the appropriate charge for both defendants was one of murder. We were not willing to accept any lesser charge based on the defendants' mental health difficulties, so the case was rightly taken to trial.

"The male defendant accepted his guilt on the first day of trial. The other defendant maintained her not guilty plea but, after the jury heard all the medical evidence presented they concluded that any difficulties that she may have had did not prevent her forming an intent to kill. She was therefore convicted of the murder of both victims.

"Our deepest sympathies are now with the extended family and friends of Elizabeth and Katie Edwards as they attempt to come to terms with this horrific crime."