The call to action [CTA] feature had been popular with broadcasters, who used it as a way to steer users to their own sites.

The change in policy was made earlier this month without warning.

Experts say the decision is another illustration of the power that Facebook now has over how the public reads and watches the news.

According to a recent study, 28% of people in the UK use Facebook as a news source at least once a week, with that figure rising to 41% for the under-35s.

Call-to-action cull

The BBC noticed the CTA facility was no longer available on video upload pages on 9 June, but was only able to get confirmation this was not a bug 12 days later.

"After considering a number of factors, we have removed the option to add a CTA to the end of native video," said a spokeswoman for the social network.

"We're exploring other ways for partners to achieve similar objectives, and will have an update in the coming months."

Video-makers can still add a link to the text that appears at the top or bottom of native video posts. However, this does not appear if the video is being watched in full-screen mode, and will therefore be missed if a user is allowing one clip to auto-play after another.

A "click for more" link does still appear superimposed over videos viewed on PCs.

However, it now makes the clips appear larger rather than directing users to third-party websites, as had been the case before.

Many broadcasters - including the BBC - upload shortened versions of their material in order to direct audiences to the full versions on their own sites.

Others, such as al-Jazeera's AJ+ service, are content to build awareness for their brands by making clips for the social media platform without trying to send users to their sites.

Facebook itself has an incentive to discourage audiences from leaving as this allows it to show them more ads.

"This is further evidence that having eaten the audiences for newspapers, Facebook is now keen to stifle the audiences for broadcasters," commented Roy Greenslade, professor of journalism at City University London, and a former editor of the Daily Mirror and Sunday Times.

"It is now the most dominant media platform on the planet.

"My great fear is that in squeezing newspapers and now broadcasters, Facebook will eventually kill the golden goose - mainstream media - that lays the golden egg - content."

While Twitter still allows users to add CTA links to its video uploads, Snapchat has never done so for clips posted to its Discover section.

One expert said Facebook might prefer to share some of the revenue it generates rather than send audiences elsewhere.

"Facebook has the power in its relationship with publishers," said Joshua Benton, director of the Nieman Journalism Lab at Harvard University.

"It has the audience loyalty, it has the attention and it wants to maximise that.

"Publishers are in a position where they have to evaluate in each case whether the trade-offs make sense or not.

"Everyone is anticipating Facebook will come forward with better ways to monetise videos - ads could be inserted between them - and it will probably try to make that relatively generous [to the video creators] at first because it will want publisher buy-in.

"But we're certainly in a world where Facebook and maybe a few other companies in northern California have a huge amount of power in terms of audience."

(BBC)