A Turkish journalist has been sentenced to more than a year in jail for her work on the Paradise Papers investigation into offshore tax havens, because it revealed details of the business activities of the country’s former prime minister and his sons.

Pelin Ünker, a member of the International Consortium of Investigative Journalists (ICIJ) was found guilty in an Istanbul court of “defamation and insult” for writing about companies in Malta owned by Binali Yıldırım and his sons. Yıldırım was prime minister from 2016 to 2018, when the post was abolished, and is now the speaker of the country’s national assembly.

After the sentence was issued, Ünker told the ICIJ she intended to appeal, pointing out that the Yildirim family had admitted that articles about their Maltese businesses were accurate.

She said: “This decision is not a surprise for us. Because the result was certain from the beginning. There is no criminal offence or defamation in my articles.

“The fact is Binali Yıldırım’s sons have Maltese companies. Binali Yıldırım had already accepted that they have these companies. In the indictment, it is also accepted.”

Turkey has the world’s worst record for jailing journalists, with 68 in prison at the end of last year, according to the Committee to Protect Journalists. All the jailed reporters are facing charges of crimes against the state.

The Paradise Papers revelations stemmed from a mass leak of documentation on the offshore financial industry published by a consortium of 90 media outlets around the world, including the Guardian. The investigation has sparked new or expanded criminal investigations in Switzerland and Argentina and accelerated the process of reform in the European Union.

The ICIJ’s director, Gerard Ryle, condemned Ünker’s jail sentence of 13 months, as the latest in a long series of attacks on free speech in Turkey.

“This unjust ruling is about silencing fair and accurate reporting. Nothing more,” Ryle said. “ICIJ commends Pelin Ünker’s brave and truthful investigative reporting and it condemns this latest assault on journalistic freedom under Turkish president Recep Tayyip Erdoğan’s autocratic rule.”