Posts containing the word “vote” have been blocked from appearing on Facebook, raising fears about censorship, Sky News can exclusively reveal.
The issue appears to have affected people in the UK using Safari and Chrome browsers.
However posts on the social media giant’s app were unaffected.
Tests by Sky News, with several accounts, produced a red error message, saying: “Your last post wasn’t published. Please try again.”
In other experiments, no error message appeared, but the post was not published – remaining invisible to the user, although it was visible to their friends.
Posts not containing the word “vote” appeared as normal.
Those by Sky News with the word “vote” began to be published around 7.30pm on Wednesday.
A Facebook spokesperson said it was looking into the issue and suggested the problem was most likely a temporary mistake in the social network’s code.
Known in technical jargon as a bug, these faults are common, and are often linked to software updates.
Without this information, Facebook users who encountered the problem were concerned their voices were being suppressed.
“I was really worried,” said Dan See, a Facebook user who posted “Don’t vote for Boris Johnson” after watching the leaders’ debate on ITV.
He added: “I felt like someone was watching me. It was really intrusive. It made me feel censored.”
Mr See’s post initially appeared on Facebook in the hours after Tuesday evening’s debate between Prime Minister Boris Johnson and Labour leader Jeremy Corbyn, but when he checked in the morning it had disappeared.
“I couldn’t see it on my profile or any of the notifications about it,” he said.
Further posts, including “Don’t vote for Jeremy Corbyn” and “Make sure you vote”, also failed to appear, leading Mr See to believe the issue was the word “vote” itself.
It is not known how many people were affected, but in the 24 hours following the first leaders’ debate, several UK-based Facebook users raised concerns on Twitter.
“My Facebook won’t let me post anything on my wall with the word “vote” in it. Weird,” wrote DJ Paperwork at 7.18am.
“If I post the word “vote” on Facebook, the post will not appear. What is this fresh hell?” said Lucy Lepchani at 8.25am.
“Whether bug or policy experiment, sudden changes to platforms can have disproportionate effects on their users,” said Alex Krasodomski-Jones, director of the Centre for the Analysis of Social Media.
“Platforms are under huge pressure to do more towards electoral integrity, but moments like this should remind us of the real crisis in legitimacy and oversight we still need to resolve.”