Researchers at Belgium's KU Leuven university and New York University examined 33.8 million Facebook ads that ran on the social media site between July 2020 and February 2021.

"This is the first known study to quantify the performance of Facebook's political ad policy enforcement system at a large and representative scale," the team said in a summary of their findings.

Facebook imposes stricter conditions on paid advertisements that concern "social issues, elections or politics," including posts that promote particular candidates.

Ads labelled as political appear on the site with a disclaimer that explains who paid for them. Ads that are found to be political, when they were not declared as such, are taken down.

But the researchers found that in 189,000 cases when Facebook reviewed an ad to check whether or not it should be treated as political, it was wrong 83 percent of the time.

These included 117,000 cases when Facebook's detection system failed to flag up ads that should have been treated as political, and 40,000 ads that were mistakenly flagged as political when they were not.

The researchers noted that Facebook's enforcement of the policy relies heavily on detecting keywords in ads under an automated system, although staff also play a role in moderating the content.

The period studied included elections in two large Facebook markets, the US and Brazil, and the researchers warned that mislabelling the ads created opportunities for manipulation.

"When Facebook fails to identify advertisers who do not properly declare their ads as political, those who are malicious can spread disinformation," they warned.

"Users who see ads without a political disclaimer may not be aware that their intent is to influence them," they added.

The team noted that the US social media giant missed a higher proportion of undeclared political ads outside the United States.

"The platform had the worst record in Malaysia, where it missed as much as 45 percent of ads from obviously political pages or advertisers," the research summary said.

"In Macedonia, Argentina, Turkey, Portugal, France and Serbia, Facebook missed up to one out of four ads from such pages, which were sponsored by candidates or parties."

The study follows criticism of Facebook over a recent whistleblower scandal, including accusations that the site's ability to tamp down hate speech and misinformation has been seriously lacking outside the West.

Within the US, Facebook introduced a moratorium on political ads around the 2020 presidential election, following deep controversy over the platform's role leading up to the 2016 election of Donald Trump.

Facebook nonetheless allowed more than 70,000 political ads to run during the 2020 moratorium, the researchers found.

AFP has reached out to Facebook to request a comment on the study.