The rise of fake news, attacks on the media by the administration of Donald Trump, and bad behaviour by some parts of the press have created a uniquely perilous time for journalism, according to former Sunday Times editor Sir Harold Evans.

Evans was speaking before a Bureau of Investigative Journalism event about reporting in a post-truth world. Evans said this was a world that was epitomised by the behaviour of Trump and those around him.

“In terms of truth of journalism it is a very perilous time,” said Evans. “We have those people who don’t have the brains to distinguish facts [from fiction]. Then we have the bad performers in the press, particularly numerous in the UK … Then you have got the assault [on the media].”

He later added that the combination of factors meant the media had “never known a worse situation than this”, and further consolidation of news outlets in the hands of moguls such as Rupert Murdoch would make it harder to combat the threat.

Evans said Trump was a “congenital liar” who lied even when there appeared to be no advantage in doing so. “He is in a class by himself as a liar; it’s so often, and so intense and so astonishing.”

The attempts by Trump’s close adviser Steve Bannon to brand the media the “opposition party” were designed to imply they were “bound to lie” and “therefore truth is to be ignored”, he said.

Evans was editor of the Sunday Times for 14 years, setting up the Insight investigative team and overseeing numerous scoops including the paper’s revelations about birth defects caused by Thalidomide.

After Murdoch’s acquisition of Times newspapers in 1981, he was made editor of the daily title, but resigned shortly afterwards over what he has said were political differences with the new owner.

Evans said Murdoch, who has developed a close relationship with Trump, would not be controlled by him.

“Rupert is too intelligent to become a captive of Trump, but he may prefer Trump to anybody else. Murdoch is much more a strategic thinker, more intelligent in public affairs than Trump.”

However, he said he opposed Murdoch’s bid to take full control of Sky through a takeover by 21st Century Fox, on the grounds that concentration of media reduced incentives to tell the truth.

“My general view is that freedom or truth is more likely to flourish when it is in a competitive arena, and not likely to emerge when the agents of discovering truth are concentrated.

“The temptations of monopoly are great. I am opposed to the conglomerates. It isn’t just Mr Murdoch to blame for all of this. It’s the nature of large monopolies, [they are] less concerned with veracity.”

Evans added that bad behaviour by sections of the media, particularly in the UK, were undermining trust in the rest of the industry.

“The press is condemned by the worst performers. The worst liars, the worst cheats the worst scandals,” he said.

In that environment, transparent investigative journalism was more important than ever.

“If I think back to the hacking scandal, and the lies that were told there, they were exposed in the end by a piece of investigative journalism. There are still enough rational people around to recognise investigative journalism is more important than ever.”