Tweets from accounts that you do not follow will now appear in your Twitter timeline, the social network has confirmed.
The new feature has been added to the company's support page after users first reported the appearance of content from other account last week, with many believing it to be just an experiment.
Normally, a user's Twitter timeline is filled by posts from accounts the user follows, as well as retweets from those accounts.
But this has now changed, as the site's support page explained.
"Additionally, when we identify a Tweet, an account to follow, or other content that's popular or relevant, we may add it to your timeline," a statement on the official site said.
"This means you will sometimes see Tweets from accounts you don't follow. We select each tweet using a variety of signals, including how popular it is and how people in your network are interacting with it. Our goal is to make your home timeline even more relevant and interesting."
The new feature will also mean that favouriting tweets - traditionally a way of saving a tweet without making them public, will start to appear in users' timelines as well. But it has always be possible to view favourites by using the Discover section of the Twitter app or website.
The change is seen by some as an attempt to increase user interest and engagement in the wider content of Twitter, as the site looks to close the gap on Facebook in user numbers.
As of March this year, Twitter had 241 million active users globally, with more than 15 million in the UK. In comparison, rival social network Facebook has more than one billion users active each month.
The move has proved unpopular with users so far, with many taking to the site to voice their frustration at the new, unwanted content appearing within their timeline.
One user from Manchester tweeted to complain he was seeing content related to his deceased stepfather now appearing on his profile.
For the moment, the change does not affect third-party Twitter apps like Tweetbot, which are operated by independent software and present user timelines in alternative ways, as well as offering different features.
But it is likely the apps will absorb this new feature eventually thanks to their connection with Twitter's main servers.