Henry Nowak's killer has had his life sentence increased on the day his bid to reduce it failed.
After stabbing the student, Digwa lied to police that he was the victim of a racist attack.
Attorney General Ellie Reeves referred the case to the Court of Appeal earlier this year, calling the sentence "unduly lenient".
Judges at the Court of Appeal ruled on Thursday that he should serve a minimum of 25 years.
Lady Chief Justice Baroness Carr, sitting with Mr Justice Nicklin and Mr Justice Wall, said: "A minimum term of 21 years did not adequately reflect the overall seriousness of the murder. It was not only lenient, but unduly so."
She added that Digwa filming the aftermath of the stabbing amounted to "incomprehensible cruelty" which caused his victim "additional distress and degradation".
Mr Nowak's family said they faced a "life sentence of grief" as they welcomed the Court of Appeal's decision.
They said in a statement: "This decision represents an important step towards justice for Henry and our family. But no sentence, however long, can ever give Henry his life back.
"Our son was just 18 years old. He had his entire future ahead of him, a future that was so cruelly and needlessly stolen from him.
"We have been given a life sentence of grief. There is no appeal for us, no reduction in our suffering and no prospect of ever getting our son back."
Jonathan Polnay KC, for the Attorney General, earlier told the hearing that Digwa had "repeatedly" used the knife against an "unarmed and defenceless stranger".
Mr Polnay told judges that Digwa "abused the privilege" allowing Sikhs to carry blades in public and that he "zoomed in on Henry's face whilst verbally gloating" as he filmed the aftermath of the attack.
Imran Khan KC, for Digwa, previously said that the sentence already reflected these aggravating features and called for a "materially lower minimum term".
Members of Mr Nowak's family were present for the hearing at the Royal Courts of Justice in London, with one relative wiping their eyes after the judgment.
He did not appear to react as his sentence was increased.
The Attorney General said in a statement: "The brutality and cruelty of this case horrified me, and I know that feeling is shared by the British public. I welcome the Court of Appeal's decision to increase the minimum term of Digwa's life sentence.
"My thoughts remain with Henry's family and loved ones at this incredibly difficult time. No sentence can ever fill the void left by his loss, and I hope this outcome goes some way towards bringing them the justice they deserve."
The murder case sparked violent protests after police body-worn footage was released showing officers failing to give Mr Nowak first aid and instead handcuffing him as he lay dying, despite him telling officers he could not breathe.
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At the time, Judge William Mousley KC said the killer's attempts to lie to police and cover up his crime were among several aggravating factors that increased his sentence.
Mr Nowak's family say they have been "failed multiple times" by the police response to the 18-year-old's death.
In an interview for ITV Tonight, the family accused police of misleading them, giving changing accounts of their response to his stabbing and initially portraying him as the aggressor in the situation.
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