Facebook claims that it can reach more millennials and people in other demographics than actually exist in the UK, US, Australia, Ireland and France, according to census data.

In the UK, Facebook says it can reach 7.8 million users aged between 18 and 24. The Office of National Statistics, however, says there were only 5.8 million people in that age group in the whole in the country in 2016.

The problem appears to be systematic and global, with similar discrepancies having been found in various different countries around the world in Facebook’s key markets.

The questionable statistics were discovered by American analysts Pivotal, who noted that Facebook’s advertising tools claim a potential “reach” of 41 million Americans between the ages of 18 and 24.

According to the 2016 US Census, there were only 31 million people in that demographic in the entire country. A similar pattern holds for American 25-34-year-olds: Facebook claims 60 million people, while the census reports 45 million.

In a research note, Brian Wieser, a senior analyst at Pivotal, said that the gap is not widely known among ad agency executives. Now that its existence has been reported, he adds, it may cause large advertisers to step up demands for third-party measurement services.

“While Facebook’s measurement issues won’t necessarily deter advertisers from spending money with Facebook, they will help traditional TV sellers justify existing budget shares and could restrain Facebook’s growth in video ad sales on the margins,” Wieser said.

The problem is international. In Australia, the company claimed to be able to reach 2.3 million 20-24-year-olds in a country that only had 1.7 million of them in 2016, according to the Australian Bureau of Statistics. In France, Facebook claimed 7.4 million 18-24-year-olds even though the French National institute for demographic studies (Ined) reported 5.2 million of them 2016. In Ireland, where Facebook’s European headquarters is based, Facebook claimed 560,000 young people while Eurostat reports just 353,000 in the country in 2016.

The question marks over Facebook’s audience reach will further erode relations with advertisers still angry about ads running around fake news sites, and the social media site’s admission of a string of measurement errors affecting their campaigns inflating the average time people spend watching videos, in some cases by up to 80%.

“This is yet another self-reporting error by Facebook that doesn’t help it re-establish confidence with advertisers and the market,” said Michael Karg, group chief executive of media and advertising auditing firm Ebiquity. “They have been trying to improve things like transparency, but once again they are having issues.”

Between them, Facebook and Google are expected to take half of all internet advertising revenue worldwide, according to research firm eMarketer, and 20% of the entire advertising business.

Facebook said in a statement that its audience estimates did not match census data, but added that this was by design as ad reach numbers “are designed to estimate how many people in a given area are eligible to see an ad a business might run. They are not designed to match population or census estimates”.

The company added: “This is just an estimator and campaign planning tool. It’s not a business’ actual reach or campaign reporting, and is not billable.”

In November, Facebook launched a blog called Measurement FYI to share updates and corrections for its data. In its most recent post, the company said it was working on technology to identify accidental clicks on adverts, and announced it would begin to report the raw numbers of viewers received by adverts, before the figures are adjusted to deal with fraud, repeats, or accidentally delivered adverts.

On Wednesday, the company revealed that it had taken $100,000 (£76,300) over two years from an influence operation likely based in Russia that spread divisive social and political messages. The adverts didn’t back a particular candidate, but instead spread polarising views on topics including immigration, race and gay rights, Facebook said.

Facebook said it identified a further $50,000 in overtly political advertising that might have a link to Russia. Some of those ads were bought using the Russian language, even though they were displayed to users in English.