Leonard Downie Jr, the former executive editor of the Washington Post, has attacked online news aggregators including the Huffington Post as "parasites living off journalism produced by others".

Delivering the James Cameron Memorial Lecture at London's City University last night, Downie criticised online aggregators for filling their websites "with news, opinion, features, photographs and video that they continuously collect – some would say steal – from other national and local news sites".

The Huffington Post was founded in 2005 by socialite and columnist Arianna Huffington and earlier this year overtook the New York Times website in terms of traffic.

But Downie questioned how the blogging and aggregation site got this traffic. "Revealing photos of and stories about entertainment celebrities account for much of the highly touted web traffic to the Huffington Post... Though they purport to be a new form of journalism, these aggregators are primarily parasites living off journalism produced by others," said Downie, who edited the Washington Post for 17 years until 2008 and is now the paper's vice-president at large.

Other leading US web news aggregators include the Drudge Report and NewsNow. Downie criticised them for attracting audiences by appealing to what he claimed are predictable sets of political prejudices on the left or the right, "along with titillating gossip and sex".

Downie also used the lecture, named after the late war correspondent and broadcaster, to criticise a journalistic culture which has led to "tabloid invasions of privacy". And he hit out at broadcasters and publishers which allowed "news [to be] presented as entertainment and entertainment presented as news".

He added: "Credible, verifiable journalism about what is important in life is needed more than ever amidst the babble of the blogosphere and social networks."

He also said the internet had the power to dramatically improve journalism. "The best journalism being produced now – thanks to the same forces of change that have so disrupted the old order – is arguably better than ever."

"Journalists can gather news and information much more widely and deeply on the internet. They can update and supplement their reporting continuously on blogs and social media – and they can have their reporting enriched and fact-checked by their audiences."

But he said news organisations must find new ways of funding their output and be prepared to collaborate with rivals, in print and online, in order to survive.

Downie contrasted news aggregators with other websites founded by former journalists, members of the public or recently established charitable organisations set up to fund "accountability journalism". He praised them for filling a gap left by America's big newspapers, many of which are shrinking in size and facing unprecedented financial challenges.

"American journalism is at a transformational moment," he said, "in which a long era of dominant newspapers and influential network television news programmes is rapidly giving way to a new journalistic era in which both the gathering and distribution of news is more widely dispersed."

He pointed out that despite the systemic and structural changes facing the industry, Americans spend more time consuming news than they did a decade ago.

Describing a "transformational moment" in American journalism, he said: "A long era of dominant newspapers and influential network television news programmes is rapidly giving way to a new journalistic era in which both the gathering and distribution of news is more widely dispersed.

"The challenge I see – in the United States and elsewhere, over time – is to turn this tumultuous moment of transformation into a beneficial reconstruction of journalism, enabling credible, verifiable, independent news reporting to emerge, enlivened and enlarged, from the current decline of long-dominant news media."

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