A 15-year-old girl has been found guilty of murdering a mother and daughter who were stabbed through the throat as they slept.

The girl, who cannot be named for legal reasons, admitted to the killing of dinner lady Elizabeth Edwards, 49, and her 13-year-old daughter Katie Edwards in Spalding, Lincs, in 2015 but denied murder 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 deliberation.

She was accused of planning the murder with her former teenage boyfriend and carrying it out "to the letter" when they were both 14.

The girl's former boyfriend, who also cannot be named for legal reasons, admitted murder on the first day of the trial.

The pair are believed to be the youngest couple to be convicted of murder in Britain.

During police interviews, the female defendant described how her 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.

Post-mortem examinations found Katie died as a result of being stabbed twice in the neck, while the mother had knife wounds on her hands, believed to be as a result of her trying to defend herself from the attack.

While giving her confession, the girl told police the plan to commit murder initially started as a joke between the young couple.

During the trial, consultant forensic psychiatrist Dr Philip Joseph told the court the crimes did not have any connection to mental illness but instead was the result of a toxic "us against the world" mentality between the pair.

Comparing the teenagers to Bonnie and Clyde, the infamous US crime duo Bonnie Parker and Clyde Barrow, Dr Joseph told the court: "If they hadn't got together and had the intense, toxic relationship, they [the killings] would never have happened.

"She 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."

After committing the murders, which the girl described the police as a "a breeze" during interviews, the court heard that the pair had a bath together and watched four Twilight films.

Both the boy and girl now face an indefinite detention when they are sentenced at a later date.

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

DCI Martin Holvey, of Lincolnshire Police, said: "Between 13 April and the 15 April this year, a tragic event happened in Spalding, Lincolnshire, whereby a mother, Elizabeth Edwards and her young 13 year-old daughter, Katie, were brutally stabbed in their own home as they slept.

"This tragedy was distressing for all the families involved, for Katie's school friends, for Elizabeth's colleagues ,friends and children at the school where she worked, as well as having a shocking impact on the wider community of Spalding.

"What makes this case even more shocking is that these two were aged just 14 when they planned and committed these callous, senseless, and unprovoked attacks on Elizabeth and Katie.

"This case has left a number of lives in ruins, not only Elizabeth and Katie's families but also the two juveniles who committed this horrendous crime."