Vickrum Digwa, the killer of Henry Nowak, should have received a longer sentence because of his false accusations of racism against his victim and his abuse of a religious right to carry a knife, the court of appeal has been told.

Digwa, 23, was jailed in June for life with a minimum term of 21 years for murdering Nowak, 18, in an attack in Southampton in December last year.

When police arrived at the murder scene, Digwa falsely claimed Nowak had racially abused him and knocked his turban off, causing officers to arrest and handcuff Nowak before they saw his fatal injuries. The incident led to riots in Southampton.

The attorney general, Ellie Reeves, has referred Digwa’s sentence to the court of appeal as “unduly lenient”. On Thursday the court also heard Digwa’s appeal that his sentence was “manifestly excessive”.

In written submissions on behalf of Reeves, Jonathan Polnay KC, said the sentence failed to take into account a series of “significant aggravating factors”, including Digwa’s “false allegation of racism against the deceased”.

Polnay added that Digwa had “abused the privilege” afforded to Sikhs to carry a knife in a public place for religious reasons.

He said: “The offender was carrying a substantial bladed weapon in public, immediately available for use, which he repeatedly deployed against an unarmed and defenceless stranger. He abused a statutory religious exemption permitting him to carry that weapon.”

At Digwa’s sentencing hearing, Judge William Mousley KC said that on the night of the murder, Digwa had a small knife hidden from view called a kirpan, which is a strict requirement for Sikhs to have at all times.

Digwa also had a “large dagger in a sheath”, due to his membership of an order of Sikhs called the Nihang, who have a tradition of having a second knife.

Polnay told the court that the attempt to increase the sentence was “not about the lawful possession of a kirpan” and the laws allowing Sikhs to carry blades, adding: “It is about what followed.”

He said that Digwa’s use of the dagger was, “on at least one level, analogous to an abuse of trust”.

He continued: “When a person who has the benefit of that accommodation uses the article protected by law for repeated lethal violence, that is an aggravating factor of particular weight.”

Polnay also cited “the offender’s sustained attempts to pervert the course of justice” and “his procurement of his mother’s assistance in concealing evidence”.

Digwa’s mother, Kiran Kaur, 53, was jailed for three years for assisting an offender by taking the knife from where her son murdered Novak back to the family home.

Polnay said other aggravating factors included Digway’s “degradation of the dying victim by filming him, and his conduct in misleading responders”.

His submission said: “The minimum term imposed failed adequately to reflect the combined weight of those features.”

Digwa watched proceedings from HMP Frankland in County Durham, wearing a dark suit, shirt and tie.

Imran Khan KC, for Digwa, said the sentence could be increased only in “exceptional, rather than borderline cases, and directed to gross error by the sentencing judge”. He asked: “Where is the gross error?”

He acknowledged the law allowing Digwa to carry the knife “plainly did not” allow him to use it to murder Nowak, but also said the sentencing judge had “already expressly aggravated [the sentence] for both the dangerous weapon and the abuse of religious privilege”, as well as other factors.

Khan said: “There must be vigilance for using the same underlying facts repeatedly … Simply repeating the points and putting them into subheadings does not make the argument any better.”

Khan also said carrying a knife for religious reasons “should not of itself” cause Sikh offenders to be given longer sentences.

Challenging the length of the sentence, Khan said that, while Digwa’s lies to first responders “unquestionably affected the dignity with which Mr Nowak was treated”, his wounds were unsurvivable.

“The false account therefore did not worsen the fatal injury or deprive Mr Nowak of the prospect of survival,” he said.

When the lady chief justice, Sue Carr, responded that it “must have been terrifying for Henry”, Khan said: “I accept that.”

The hearing before Lady Carr, Mr Justice Nicklin and Mr Justice Wall is due to conclude on Thursday.