Craig Roy was plagued by guilt after cheating on his boyfriend with his victim, 16-year-old Jack Frew, court heard

A teenager has been found guilty of murdering a schoolfriend he claimed had pestered him for sex.

Craig Roy stabbed 16-year-old Jack Frew 20 times and slashed his throat with a kitchen knife in woodland near their school in East Kilbride, South Lanarkshire, in May 2010.

Roy had cheated on his long-term boyfriend with Jack and was plagued by guilt over his infidelity, the trial at the high court in Glasgow heard.

Roy, 19, admitted stabbing Jack but denied murder, claiming he had no memory of the incident and that he had taken a knife with him to scare his victim.

Roy said he recalled taking out the knife and then seeing Jack lying bleeding to death on the ground. Instead of calling an ambulance, Roy called his boyfriend for help.

Roy claimed Jack had been blackmailing him for sex after they shared a sexual encounter in the school toilet three months before the murder. He said the schoolboy, described by friends as flirtatious and flamboyant, was a "sex pest" who had threatened to tell Roy's boyfriend he had cheated.

Roy was the only person in the three-week trial to make the claims. Jack suffered punctured lungs, a cut windpipe and knife damage to his ribs in the fatal attack.

The defence said Roy had a personality disorder that diminished his ability to control his actions. But the jury of nine women and five men rejected this claim, finding him guilty of murder after almost two hours of deliberation.

Roy will be sentenced on 1 March at the high court in Edinburgh.

Janet Cameron, the procurator fiscal for Lanarkshire, said: "Craig Roy armed himself with a knife and carried out a violent, sustained and murderous attack on Jack Frew. This crime yet again demonstrates the devastating consequences that can follow from carrying a knife.

"The senseless loss of such a young life is tragic and caused shock and concern not just to the local community in East Kilbride but across our country. Today's conviction should act as a warning to anyone thinking of leaving home with a knife or using a knife that prosecutors, working together with the police, will ensure that those people are caught, prosecuted and brought to justice."