Professor Richard Tait, former Editor-in-Chief of Britain's national Independent Television News network and now Director of the Centre for Journalism Studies at Cardiff University, UK issued the warning at a lunchtime briefing for Assembly Members, researchers and civil servants.
"Voter turn-out fell to 38% in last year's Assembly elections, and nationwide surveys suggest only 19% of the public in the UK now trust politicians to tell the truth," said Professor Tait. "We are witnessing a meltdown of public trust, which must be addressed urgently.
"If trust in politicians and participation in elections continue to decline, it is no exaggeration to say that the whole democratic process is under threat.
"The media are also in trouble," he added. "Journalists are regarded as even less trustworthy than politicians, and only 6% of the public see newspapers as the most fair and unbiased source of news."
While research showed only 13% trusted journalists to tell the truth, 71% trusted newsreaders to do so - even though newsreaders were usually journalists.
Trust in institutions was also a cause for concern, said Professor Tait. While 71% trusted the Post Office, 53% trusted schools, only 22% trusted the civil service and 13% trusted the Scottish Parliament and Welsh Assembly.
Asked whether the government of the day would put the national good above party gain, 39% believed so in 1974, but by 2002 the figure had fallen to 16%.
This lack of trust is leading to a lack of engagement, especially among the young, he warned. "Only 16% of voters under 25 took part in the last election," he said.
"With an increase in so-called special advisers and the politicisation of the British civil service," the signs are that this situation will worsen.
Professor Tait is a member of the Independent Review of Government Communications, chaired by Guardian Media Group Chief Executive Bob Phillis, which was commissioned by the Government after the Jo Moore/Martin Sixmith case, which highlighted the conflict between civil servants and Government-appointed advisers.
The review reported its findings this week.
His presentation will drew on the committee's report, as well as on research from the School of Journalism, Media and Cultural Studies at Cardiff University, including studies of media coverage of the 2003 Welsh Assembly elections and the Iraq war.
This was the latest in a series of expert briefings to the Assembly by academic staff at Cardiff University.