Freelance security journalist Robert Verkaik said that he assumes that anything on his telephone and computer is potentially available to the security services.
And he has revealed that since the Leveson report of November 2012 there has been a "huge vacuum" left because civil servants and law enforcement officers are no longer prepared to speak to journalists.
Verkaik met Mohammed Emwazi before he became known as the terrorist Jihadi John.
Emwazi was one of a number of young English Muslim men who Verkaik met in 2010 because they said the security services were harassing them. Emwazi, who was filmed beheading a number of hostages including the journalist James Foley, is believed to have been killed in a US air strike in November 2014.
Verkaik has written a book – Jihadi John: The making of a terrorist – which attempts to piece together how Emwazi and others went from normal English teenagers, to troubled young adults to terrorists.
Talking about the current situation for UK journalists who cover security issues he said: "We are in a very difficult climate at the moment under the Terrorism Act. Whan I was writing the book I was half expecting a knock at the door before it was published."
Under the Terrorism Act journalists may be committing an offence if they do not pass on details of sources who are terrorists to the authorities.
Verkaik said: "It's a very grey area. When Emwazi was carrying out his beheadings [Met Police Commissioner] Bernard Hogan-Howe released advice to public and to journalists that if you download these videos you are committing a terrorism offence.
"That's putting journalists in a very difficult position. If we can't see what these terrorist acts are, how are we supposed to report on them? That was chilling."
Verkaik said that conversely, social media has made it easier than ever for journalists to make contact with extremists.
"Lots of IS's propaganda is through social media, and it has made it possible for the first time to speak directly to terrorists which is a whole new part of reporting in this country, and one or two journalists have been picked up just for doing that. You don't know who you're speaking to most of the time."
He added: "There's also a problem with journalists being approached by the security services to work for them. That's crossing a line – especially as journalist, as we've seen in Syria, are already target number one."
Last year BBC Newsnight journalist Secunder Kermani had his laptop seized by police because they suspected he had been in contact with terrorists.
Asked about the precautions he takes to protect himself from state surveillance, he says: "If you say you've got a source and the information they tell you has some bearing on terrorism or intelligence then you can expect a knock at the door.
"I assume that everything I do on the telephone and everything I do on the computer is available. You have to assume that. From that basis you then decide how you make notes and how you write things."
Verkaik said that the Leveson report and counter terrorism legislation had created "quite a powerful disincentive to engage with the media".
He said: "We know very little about what's going on anyway. Police officers aren't talking to us, civil servants aren't talking to us and if security sources aren't talking to us, that leaves a huge vacuum."
In recent years more than 30 public officials have been convicted (and in most cases sent to prison) for selling information to journalists. A number of police officers have also been arrested and sacked in cases where they have given information to journalists but no money has changed hands.
Verkaik said: "Post Leveson, people don't talk to you. Whitehall officials who used to feel comfortable sitting down and chatting about stuff would rather not take the risk… they would rather not have those conversations any more."
Jihadi John: The Making of a Terrorist is published by Oneworld Publications, price £9.99.
Verkaik talks more these issues, and also about how the authorities could stop people like Emwazi turning to terrorism, in the latest Press Gazette Journalism Matters podcast. Listen below, subscribe via iTunes here and via RSS here.