Facebook has restricted the ability of external political transparency campaigners to monitor adverts placed on the social network, in a move described as an “appalling look” by one of the organisations affected.

WhoTargetsMe, a British group dedicated to scrutinising adverts on the social network, has said its activities have been severely restricted by recent changes made by the social network. The change has also hit a similar programme by the US investigative journalism site ProPublica, affecting both groups’ ability to collect data on why users are being targeted by political campaigners.

The monitoring tools, which involve asking users to install a browser plug-in and collecting data on the adverts they see, has helped expose many of the advertising tactics used by politicians, making it harder for those who pay for negative adverts to escape scrutiny.

“Ten days ago, our software stopped working, and efforts to fix it have proved much harder than before,” said WhoTargetsMe co-founder Sam Jeffers. He said he feared his service could soon be in effect locked out of Facebook altogether. “Facebook is deliberately obfuscating their code. When we have made small changes, they’ve responded with further updates within hours.

“This comes in a year when over a third of the world’s population has the opportunity to vote, with elections across the EU, India, Canada, Australia, South Africa, Israel and Ukraine to name a few. In sum, they are actively trying to stop our project from gathering data about the ads they run, and the targeting of those ads. Obviously, we think this is the wrong decision.”

Facebook said the change was part of a wider crackdown on third party plug-ins such as ad blockers accessing unauthorised data from its site, although the 20,000 people who have signed up to WhoTargetsMe have chosen to share their data with the service.

“We regularly improve the ways we prevent unauthorised access by third parties like web browser plug-ins to keep people’s information safe,” said Facebook spokesperson Beth Gautier. “This was a routine update and applied to ad blocking and ad scraping plug-ins, which can expose people’s information to bad actors in ways they did not expect.”

WhoTargetsMe was founded ahead of the 2017 general election in response to concerns about the impact of online advertising during the EU referendum, when millions of pounds was spent on Facebook advertising by both Leave and Remain with little scrutiny of what voters were seeing.

Data collected by the UK organisation has helped to show how the Conservatives were focusing on personal criticism of shadow home secretary Diane Abbott during the end of the 2017 campaign, in addition to using a loophole in electoral law to campaign on local issues through targeted Facebook ads without breaking spending limits.

In addition, its tool has helped WhoTargetsMe to monitor elections around the world, with the site highlighting unusual advertising practices during Germany’s elections and in Ireland during the country’s referendum on abortion.

A similar ad monitoring tool, established by ProPublica, has also been affected by the changes. That tool had resulted in negative stories for the social network such as exposing how oil companies are sidestepping Facebook’s new ad transparency tools among other issues.

At the end of 2018, Facebook launched its own political ad archive, a move broadly welcomed by campaigners that has already resulted in additional scrutiny of political advertisers on the service including the government and obscure pro-Brexit campaign groups.

However, the social network has also been slow to give direct access to the new database to journalists and researchers, while it is currently only available in the US, UK, and Brazil – although it plans to roll it out across the EU before this spring’s European parliament elections.

Facebook has insisted its in-house transparency page is industry-leading but Jeffers said it is still “inadequate” as it doesn’t provide meaningful information about why a user is being targeted, or who is ultimately behind such advertising.