Barack Obama has pursued the most aggressive "war on leaks" since the Nixon administration, according to a report published on Thursday that says the administration's attempts to control the flow of information is hampering the ability of journalists to do their jobs.

The author of the study, the former Washington Post executive editor Leonard Downie, says the administration's actions have severely hindered the release of information that could be used to hold it to account.

Downie, an editor during the Post's investigations of Watergate, acknowledged that Obama had inherited a culture of secrecy that had built up since 9/11. But despite promising to be more open, Obama had become "more aggressive", stepping up the Espionage Act to pursue those accused of leaking classified information.

"The war on leaks and other efforts to control information are the most aggressive I've seen since the Nixon administration," Downie said in the report, which was commissioned by the Committee to Protect Journalists.

"Those suspected of discussing with reporters anything that the government has classified as secret are subject to investigation, including lie detector tests and scrutiny of their telephone and email records," the report says.

This had a chilling effect on government accountability, even on matters that were less sensitive, it said.

David Sanger, the chief Washington correspondent for the New York Times and one of 30 journalists interviewed by Downie, says in the report: "This is the most closed, control-freak administration I've ever covered."

The report said that White House officials "strongly objected" to accusations that they did not favour disclosure, and cited statistics showing that Obama gave more interviews in news, entertainment and digital media in the first four plus years iin office than President George W Bush and Bill Clinton did in their respective first terms, combined.

They cited directives to put more government data online, speed up processing of FoI requests and limit the amount of information classified as secret.

"The idea that people are shutting up and not leaking to reporters is belied by the facts," said Jay Carney, Obama's press secretary says, in the report.

In his report, Downie chronicled the Obama administration's use of the 1917 Espionage Act to prosecute leakers, and its development of a programme that requires government employees in every department to help prevent leaks to the press by monitoring the behaviour of their colleagues. The initiative, called the Insider Threat Program, was first revealed by McClatchy newspapers in June.

Under Obama, the Espionage Act has been used to mount felony prosecutions against six government employees and two contractors accused of leaking classified information to the press, including Chelsea (formerly Bradley) Manning and Edward Snowden. In all previous administrations, there had been just three such prosecutions.

Still more criminal investigations into leaks are under way, the report points out. In one of them, a Fox news reporter was accused of "being an 'aider, abettor and/or conspirator' of an indicted leak defendant, exposing him to possible prosecution for doing his job as a journalist."

The report cites the outcry in May this year, when the Justice Department informed the Associated Press that it had secretly subpoenaed and seized all records for 20 AP telephone lines and switchboards for two months of 2012, after an AP investigation about the CIA's covert operation in Yemen. Although only five AP reporters and an editor had been involved in the story, the report said, "thousands upon thousands" of newsgathering calls by more than 100 AP journalists were included in the seized records.

Following a series of meetings with journalists, after the subpoenas, the Justice Department announced revised guidelines under which investigators could subpoena and seize records.

Journalists' concerns have been compounded by the revelations by Snowden, the report said.

Jeffrey Smith, a national security reporter at the Center for Public Integrity, and one of several journalists to express such concerns, said in the report: "I now worry about calling somebody, because the contact can be found through a check of phone records or emails. It leaves a digital trail that makes it easier for the government to monitor those contacts."

Scott Shane, the national security reporter at the New York Times, said: "Most people are deterred by those leaks prosecutions. They're scared to death. There's a grey zone between classified and unclassified information and most sources were in that grey zone."

It was having a damaging effect on democracy, Shane said. "If we consider aggressive press coverage of government activities [as] being at the core of American democracy, this tips the balance heavily in favor of the government."

Downie said that while the administration provides information through social media, it is "mostly self-serving information, as opposed to information that would hold the government to account. Journalists are being told to speak to public affairs office, but the public affairs office doesn't call them back or is hostile."

The report said the Obama administration has created a climate where, even on matters not pertaining to national security, but in the public interest, government officials are reluctant to provide information, including on Freedom of Information requests.

Ann Compton, the ABC News White House correspondent who has been covering presidents since General Ford, complained that there was "no access to the daily business in the Oval Office … who the president meets with, who he gets advice from".

"He's the least transparent of the seven presidents I've covered," Compton said in the report.

The CPJ, which commissioned the study, entitled 'The Obama Administration and the Press', said: "The CPJ is disturbed that the Obama administration has chilled the flow of information on issues of great public interest, including on matters of national security.

"The administration's war on leaks to the press though the use of secret subpoenas against news organisations, its assertion through prosecution that leaking classified documents to the press is espionage or aiding the enemy; and its increased limitations on access to information that is inthe public interest – all thwart a free and open discussion necessary to a democracy."

Joel Simon, the executive director of the CPJ, said the organisation had sent the report to the president this week, requesting a meeting with the administration to address its concerns.

Simon said: "Here you have a portion of the Washington press corp affirming that this is an extraordinarily difficult administration to cover. You combine the different elements, for instance, the leak investigations, the failure to address the declassification issue, the fact that the administration has been extremely controlling in terms of access.

"Put all these together and it paints a pretty damning picture of an administration that talks about openness and transparency but isn't willing to engage with the media around these issues."

The CPJ has made several recommendations to the administration, including a call for an end to prosecutions of leakers under the Espionage Act, developing policies to limit surveillance of jounalists' communications, making good on promises to increase transparency, less restrictive responses to Freedom of Information requests, and to guarantee that journalists will not be at risk from prosecution for receiving confidential and/or classified information.