Julian Assange’s detention “sets a dangerous precedent for journalists”, according to politicians from the Council of Europe’s parliamentary arm, who voted on Tuesday to oppose the WikiLeaks founder’s extradition to the US.

The words of support for Assange and implicit criticism of the UK government will be contained in a final report produced by the Labour peer Lord Foulkes for the Parliamentary Assembly of the Council of Europe, which focuses on upholding human rights across the continent.

Assange is being held in London’s Belmarsh prison prior to an extradition hearing that will begin in February. A US grand jury has indicted him on 18 charges – 17 of which fall under the Espionage Act – around conspiracy to receive, obtaining and disclosing classified diplomatic and military documents.

Foulkes had drafted an initial report – Threats to Media Freedom and Journalists’ Security in Europe – that will now contain amendments referring to Assange tabled by a number of European parliamentarians.

One of the amendments backs the recommendation of the UN special rapporteur on torture who called last year for Assange’s release and for extradition to the United States to be blocked. The other states that his possible extradition to the US “would set a precedent and threaten journalists’ freedoms in all member states”.

Foulkes told the Guardian that campaigners and supporters of Assange had written to him while he was writing the report, which addresses media freedoms and threats to journalists in countries including Russia, Turkey and Malta, and asked that he consider including an amendment mentioning Assange.

As a rapporteur for the assembly, he said it was not his role to do so but that colleagues from other states had done so.

He added: “I was in favour of him being sent back to Sweden when there were allegations against him to face, but as far as the US is concerned I think there would be deep concerns if he were to be sent there.”

While the report is non-binding on the UK or on British courts, Assange’s supporters are likely to cite it as a moral weight in their campaign to stop his extradition.

If convicted, Assange faces a prison term of up to 175 years.