More than two dozen prominent writers, including David Hare, Tom Stoppard, Hari Kunzru, William Boyd, Ali Smith, Sarah Waters and Monica Ali, have called on David Cameron to urge the Turkish government to halt its crackdown on freedom of speech, the Guardian reports.

The campaign by writers’ associations English PEN, Wales PEN Cymru and Scottish PEN comes amid growing persecution of journalists, writers and publishers in Turkey, with hundreds arrested, prosecuted or sacked over the past year.

In a letter to the prime minister, signed by 25 writers, the PEN branches warn that “over the past five months, intimidation, threats and even physical assaults against journalists, writers and publishers have become the norm”.

It asks Cameron to urge the Turkish prime minister, Ahmet Davutoğlu, who is visiting London next week, “to ensure that his government acts in accordance with Turkey’s obligations to respect the right to freedom of expression”.

Kunzru, one of the letter’s signatories, said the Turkish government had “revealed a deeply authoritarian streak”.

He added: “Absurd cases like the public health official prosecuted for a Facebook post comparing Erdogan to Gollum show a kind of sensitivity and vanity that suggests an incipient cult of personality.”

Among the incidents of intimidation highlighted by PEN, the Guardian says, are two attacks on the offices of the newspaper Hurriyet, including one supported by a serving Turkish MP; the raid and seizure of Koza Ipek Media, known for being critical of President Recep Tayyip Erdoğan; and the detention of three journalists working for Vice News. The letter warns that “independent journalism is now on its knees”.

The authors also raise concern for Can Dündar, editor-in-chief of Turkish daily Cumhuriyet, and Erdem Gül, the paper’s Ankara bureau chief, who were arrested on espionage and national security charges on Nov 26.

The charges against the pair, who continue to be held in pre-trial detention, relate to an article and video published on the Cumhuriyet website on May 29, 2015 about Turkey’s intelligence agency providing arms to Islamist rebel groups in Syria.

The authors wrote: “We are appalled that both journalists may face a maximum life sentence for the charge of divulging state secrets, a sentence of 20 years for espionage, and 10 years for membership of a terrorist organization, for a story that the authorities appear to have accepted is true.”