The Old Bailey jury trying a teenager accused of murdering promising young footballer Kiyan Prince was discharged today after "an incident" away from the court.
The jury had been considering its verdicts for two days when one of them reported the matter to the court.
The Recorder of London, Judge Peter Beaumont, ordered a retrial.
He said: "The jury was discharged as a result of an incident having taken place away from court and during an adjournment during the course of the jury's deliberations."
A new trial will take place at a date to be fixed.
A first jury could not agree when the 16-year-old appeared earlier this year and the current retrial was arranged.
The youth, who cannot be named for legal reasons, has admitted stabbing Kiyan on May 18 but denies it was murder.
Kiyan's parents, boxer Mark Prince and Tracey Cumberbatch, have sat at the side of the court during the case.
Nicholas Hilliard, prosecuting said a play-fight outside Kiyan's school gates ended in his death.
Kiyan, 15, who played for Queens Park Rangers' youth team, intervened in a fracas which turned fatal when the 16-year-old produced a knife, the jury was told. He was stabbed in the heart.
Kiyan was in Year 10 at the London Academy, Edgware, and was described by a teacher as a "gorgeous boy, very popular and well liked by all who knew him".
The judge said the incident "plainly was not the fault of the defendant".
An application was made by the defence for the defendant to be retried, which the judge granted.