Lord Black of Brentwood described the implications of a royal charter on press regulation as a "chilling prospect" in a speech to the Scottish Newspaper Society conference in Glasgow.
The executive director of the Telegraph Media Group has overseen the development of the Independent Press Standards Organisation (Ipso), which is set to replace the Press Complaints Commission in June.
It follows the phone-hacking scandal that led to the closure of the News of the World in 2011 and the Leveson Inquiry into press standards.
However a Government-backed royal charter on press regulation, which sets out legal requirements, was granted in October after the newspaper industry lost a court challenge.
In a speech at the same conference, First Minister Alex Salmond defended the Scottish Government's support for the controversial royal charter, but also acknowledged the new industry body. He said: "My view is that Ipso will go actually a long way to meeting Lord Justice Leveson's proposals for independent self regulation."
Lord Black said: "There is a clear and present danger to press freedom in this country. It's vital that all the institutions which represent our industry continue to fight our corner with vigour."
He described the royal charter as a "menace" and said it represented an "unacceptable infringement of press freedom and freedom of expression".
He said: "What has happened is that politicians have laid out how they expect the regulation of the press to be organised and they now have the tools to make that compulsory if they choose to do so. That involvement... could easily take politicians and governments to the heart of the newsroom and what you can and cannot publish.
"That for me is an incredibly chilling, authoritarian prospect hiding behind the facade of an arcane constitutional document signed by Her Majesty the Queen.
Mr Salmond said the royal charter had been given a mandate by a unanimous Holyrood vote.
He said he backed proposals to create an "independent recognition panel" to assess whether its requirements had been met.