Israel’s Supreme Court has doubled a nine-month prison term given to a border police officer who shot dead an unarmed Palestinian teenager.

Two of three judges voted to increase Ben Dery’s sentence for killing Nadim Nuwara, 17, at a protest near the occupied West Bank city of Ramallah in 2014.

Dery was jailed in April for causing death through negligence, a charge that was downgraded from manslaughter under a plea agreement. He was also fined 50,000 shekels (£10,700).

Dery shot dead Nadim during a demonstration outside Israel’s Ofer prison, near the Palestinian town of Baituniya.

A second teenager – Muhammad Abu Thahr, 16 – was also killed during the demonstration but Israeli prosecutors said there was insufficient evidence to pursue charges over his death.

The youths were shot dead at a Nakba day protest, held annually to commemorate the displacement of more than 700,000 Palestinians during the 1948 war.

Israel initially insisted its forces fired only rubber bullets after they were pelted with rocks by Palestinian youths. But it later emerged Dery had fired live rifle ammunition at Nadim, fatally wounding him in the chest.

The killings sparked an outcry after CCTV footage appeared to show the two boys were shot despite posing no immediate threat to Israeli forces.

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The lenience of the nine-month jail term imposed on Dery prompted further anger. The judge who sentenced him said he was “an excellent police officer who was conscientious about orders”.

Under Israeli law, death through negligence typically carries a prison sentence of between eight and 20 months.

“We are not surprised by the ridiculous sentence,” Nadim’s father, Siam Nuwara, told Haaretz at the time. “As soon as the plea agreement was signed we knew that this was the direction.

“We are dealing with an entire system that discriminates on the basis of race and arrives at decisions that are far from just.”