The family of Palestinian activist Nizar Banat, who died in Palestinian custody in June, stepped up its quest for international justice on Thursday, turning to British police and the UN.

Banat -- a leading critic of the Palestinian Authority and its 86-year old president Mahmud Abbas -- died after security forces stormed his home in the flashpoint city of Hebron and dragged him away.

A post-mortem found he had been beaten on the head, chest, neck, legs and hands, with less than an hour elapsing between his arrest and his death.

Banat's family has said it has no confidence in the PA's capacity to deliver justice, and called for an international probe.

A statement from the family's lawyers, the British firm Stoke White, said they have asked Britain's Metropolitan Police to open an investigation under the principle of universal jurisdiction.

For a small number of serious offences, Britain's courts can hear cases even if the alleged crimes were committed abroad.

Stoke White also said it had asked multiple branches of the United Nations human rights system to open investigation, including the Working Group on Arbitrary Detentions and four special rapporteurs.

The death in custody of Nizar Banat, a leading critic of the Palestinian Authority, sparked angry demonstrations in the West Bank to demand justice
The death in custody of Nizar Banat, a leading critic of the Palestinian Authority, sparked angry demonstrations in the West Bank to demand justice AFP / ABBAS MOMANI

Ghasan Khalil Banat said his brother's "murder" was a "tragedy for our family, but also a tragedy for the Palestinian people."

"The so-called investigation that was carried out into his murder is an embarrassment and the PA should feel ashamed of it," he said in the statement.

The head of international law and Stoke White, Hakan Camuz, said: "Responsibility for the murder of Nizar Banat very clearly lies with the senior leadership of the Palestinian Authority including President Mahmud Abbas and Prime Minister Mohammed Shtayyeh."

Shtayyeh and the PA have promised accountability over Banat's death.

Camuz accused the PA of a long-standing bid to silence dissent.

"They cannot be allowed to get away with this and this is why we are submitting these complaints and petitions to the British police and the UN," he said in the statement.

The UN and the European Union this week raised alarm over a spate of arrests of activists by Palestinian security forces since Banat's beath, warning the PA appeared to be cracking down on basic freedoms across the West Bank, a territory occupied by Israel since 1967.