Stories aren't the only thing that high-profile studio anchors at CNN apparently know how to break. Judging by the latest TV ratings, they also seem to be uncannily successful at destroying the loyalty of viewers.

Figures from ratings agency Nielsen show that America's most famous rolling news brand has just experienced its worst month for almost 20 years, parting company with more than 50 per cent of its audience in 12 months.

The development follows years of unrelenting decline for the network, which pioneered 24-hour news in the 1980s, and was for years the top-rated news channel. It has in recent years suffered intense competition from Fox News and MSNBC, which both now outperform it in almost every time slot.

The figures also raise questions about the future of Piers Morgan, hired to shake up its prime-time schedule with an hour-long weeknight interview show. He drew an average of 417,000 viewers, a fall of 50 per cent and the network's worst figure for the slot since the early 1990s. Morgan's show is largely admired by critics. But his audience is volatile, and seems to vary according to the calibre of guests. In early 2011, when he took over from octogenarian Larry King, who boasted around 600,000 viewers, Morgan said his show should be judged "by how we settle down in between six months and a year". By that measure, he's in trouble.

May saw CNN's average audience fall to 388,000, of which a mere 113,000 are adults in the 25-54 age bracket that advertisers covet. The figures represent an exponential increase in the rate of decline for the channel, which is also undermined by the internet's rise in popularity as a breaking news source.

They also suggest that CNN is losing viewers quicker than rivals. For Fox, comparable figures (1.65m and 319,000) are down just under 10 per cent annually. MSNBC (658,000 and 213,000) is declining by around 20 per cent.

"It's really a bloodletting; there's no other way to describe it," says Robert Thompson, professor of Television and Popular Culture at Syracuse University. "The biggest problem is inherent in their brand. They are trying to stick to old-fashioned, unbiased news broadcasting when their rivals have worked out that to draw an audience when there aren't major stories breaking you need to do the opposite."

CNN, based in Atlanta, retains an expensive network of overseas bureaux and is still regarded as the nation's most reliable TV news brand. It has resisted the temptation to follow Fox and MSNBC in using partisan anchors.

As a result, CNN remains a go-to destination only when major breaking news stories unfold. May 2012 was particularly tough because it was a relatively quiet news month – whereas May 2011 saw the killing of Osama Bin Laden and major tornadoes in Missouri and Alabama.

CNN is hoping that two high-profile hires, the chef-turned-TV-star Anthony Bourdain and ABC News star John Berman will help to stem the bleeding.

Turn-offs: CNN's flop presenters

Piers Morgan

Hired in a blaze of publicity almost 18 months ago, the former Mirror and News of the World editor brings Fleet Street bombasity to his 9pm slot. But ratings for the show are highly volatile and on one night recently hit a low of just 39k in the all important 25-54 bracket.

Erin Burnett

"Smart, entertaining, and ahead of the news," is how CNN describes its 7pm anchor. The public must see things differently, given this respected (but perhaps monochrome) former financial journalist's prime-time show draws only 409,000 viewers – of which a woeful 46,000 belong to the 24-54 demographic that advertisers covet.

Anderson Cooper

Famed for his black T-shirts, perfect quiff and the live-on-air takedowns of public figures (on a slot he calls "Keeping 'em Honest!"), Mr Cooper is a cable news institution and the unofficial "face" of CNN. That hasn't stopped him managing to lose a quarter of his viewers in the past 12 months.

Wolf Blitzer