When I was at university in the 1970s, it was common to hear students screaming about living in “a police state”.

I cannot, in truth, ever remember saying so myself.

But I now think it to be the case, and that’s why I am saying so, and loudly. I have been increasingly alarmed by the actions of the Metropolitan police in their dealings with journalists over the past couple of years.

Before I present my reasons, I want to assure readers it is not special pleading on behalf of my colleagues. It is about journalists being prevented from going about their work on behalf of the public.

An outstanding example is that of Tom Newton Dunn, the Sun’s political editor, who revealed the altercation in 2012 between the then Chief Whip, Andrew Mitchell, and police officers in Downing Street.

In the Met’s official report into what became known as Plebgate, was a chilling sentence with far-reaching implications: “The telecommunications data in respect of Tom Newton Dunn was applied for and evidenced.”

To achieve that, the police utilised the Regulation of Investigatory Powers Act (Ripa), which enables an officer of sufficient rank to seize a journalist’s phone records without the journalist being informed.

It does not take a moment to realise how compromising that is for any reporter. Who is going to phone a journalist with a sensitive story, about the police or anyone else, if his/her phone records can be accessed by the police?

Fewer whistleblowers mean fewer stories. Fewer stories mean the publication of less public-interest information. Less information means an enhancement of our already secretive society.

The Newton Dunn case should not surprise us because we learned, courtesy of the US National Security Agency whistleblower Edward Snowden, that Britain’s own communications agency, GCHQ, had secretly gathered intelligence from the country’s largest telecoms companies.

Nor should we overlook the fact that many of Newton Dunn’s Sun colleagues have been arrested following the decision by the paper’s publishers to hand over documents to the Met that revealed their reporting staff’s confidential sources.

Clearly, the police have developed an appetite for knowing how the press obtain their stories, and Ripa gives them power to do so without account. Journalists’ rights to maintain confidentiality for their sources are under sustained attack.

What we don’t know is how often it is happening and the identities of the victims of snooping because the data collection is carried out in secret.

That is why an application by the Bureau of Investigative Journalism to the European Court of Human Rights to investigate British laws that appear incompatible with provisions in European law is so important.

Ripa was supposed to protect national security and detect crime while preventing disorder and protecting public health. It was even aimed at ensuring the country’s economic well-being. In others words, its scope is broad and allows for wide interpretation.

It was enacted when Britain was particularly haunted by the spectre of terrorism, and plenty of critics warned at the time that it would to be a threat to civil liberties. So it has come to pass. Journalists need to garner public support to oppose its abuse.

Roy Greenslade is Professor of Journalism, City University London, and writes a blog for the Guardian