US tech firms increasingly shape the news stories that people in the UK see and read, British media regulator Ofcom has found, leading to risks around transparency and choice in news.

In a new study of choice in news, Ofcom said Wednesday it has identified concerns around the impact of online news ‘gatekeepers’ – particularly social media, such as Facebook, but also search engines and news apps such as Apple News and Google News.

The report highlights how far these companies, which are used by two in three online adults for news, determine not only how much of the online news people see, but also how they respond to it.

In a statement, Ofcom said people value online intermediaries to help them discover news. They credit search engines with helping them to find out more about stories they had seen elsewhere, and notifications from news apps allowing them to see breaking news or stories from multiple perspectives.

But social media could have a polarising effect, the British regulator stressed, adding people who mainly use social media to access news are more likely to be less tolerant of opposing political views, less able to correctly identify factual information and less trusting of democratic institutions, compared to those who use TV and newspapers.

Other international studies support these findings — one found that users became less politically polarised if they deactivated their Facebook account for just four weeks.

And, people are unclear about the influence of gatekeepers on the news they see. Nine in ten people think that choice in news, covered by a range of organisations, is important. But people aren’t always clear about the choices that social media, search and news apps are making on their behalf, and why certain stories are shown to them or not, Ofcom pointed out.

“For example, our research shows a great deal of confusion about whether news online is personalised: 35 percent of people think it is, 36 percent think not, and 29 percent are unsure. When we explain that current ‘media plurality’ rules don’t apply to social media, search engines or news-gathering apps, people are surprised and concerned,” Ofcom said.

It then went on to add that early analysis signals new regulations may be required to understand and address the impact of online gatekeepers on media plurality.

This might include new tools to require tech firms to be more transparent over the choices they make in determining the news seen online, as well as giving users themselves more choice and control.