Robert Redford says documentaries have replaced newspapers as the media's main source of investigative journalism.
The movie star, who played Watergate reporter Bob Woodward in All The President's Men, said newspaper standards were in "steep decline".
"That's why documentaries have become so important," he told the BBC. "They are probably a better form of truth."
Redford is bringing several documentaries to the inaugural Sundance London film festival in April.
The four-day event, which is being held at the O2, will promote some of the films seen at the event's regular home, in Utah, this January.
Launched in 1978, Sundance is a showcase for independent cinema and has helped launch the careers of directors such as Steven Soderbergh and Quentin Tarantino.
It debuted two years after All The President's Men, which showed how two reporters at the Washington Post broke the story of the Watergate scandal, leading directly to the resignation President Richard Nixon.
In an extended interview with BBC Arts Editor Will Gompertz, Redford said the revelations had marked a "high point" for US newspapers.
"I came in when journalism had reached an apex of morality and professionalism, and I was very lucky," he said.
"I think it's pretty obvious it's declined since then - but I didn't imagine at the time that it would decline so steeply and drastically, that the rules that governed journalism - like you had to get two sources to go on record before you could quote them - would be gone.
"The tools of responsibility were dismissed in favour of the quick scoop. I think that has damaged journalism. The people in my country wonder, where do you get the truth?"
But Redford said film-makers had largely taken up the slack, with cinema's long production times allowing them to dig deeper than newspaper deadlines would allow.
"You can show things in documentary films that maybe a government or some big corporation might hide," he said.
"One of the films we're bringing to London is calledChasing Iceand it's a film that was made over years, documenting the reduction of ice caps, documenting how climate change is affecting the natural ecosystem of the earth.
"It's really powerful and, I think, indisputable.
"If you make a documentary that has hard evidence, that is of value to an audience. Even though there are a lot of deniers in our political system - which is insane."
Despite his belief in the power of documentaries, Redford remained sceptical of their ability to affect policy.
"Can film change anything? No. Maybe just fashion," he said.
"We delude ourselves if we think film is going to change a whole system or change a whole government."
The 75-year-old star also expressed his dissatisfaction with the current state of US politics, saying: "Our Congress is not the best and the brightest."
"The quality of intellectual exchange [is] so damaged by the behaviour of a lot of the people that are running for office. It's embarrassing, and I think our country has declined... in some cases drastically."
Over his career, which includes such classics as The Sting and Butch Cassidy and the Sundance Kid, Redford has played a politician just once, in 1972's The Candidate.
His character was a youthful, naive politician whose ideals became corrupted in the pursuit of power. Perhaps unsurprisingly, he said he had never considered running for office in real life.
"It's too narrow. You can't be your natural self, as you can see on television," he said.
"Is there any one of these people you feel is really, really natural? Unless they're so crazy they're natural."