Facebook has disabled the ability of third party political transparency advocates to monitor political advertisements placed on the social network - a move described by one of the affected organizations as an "appalling look."

UK-based WhoTargetsMe and US-based ProPublica have been using voluntarily installed browser plugins to collect data on advertisements targeting users. The tools have helped expose "many of the advertising tatics used by politicians, making it harder for those who pay for negative adverts to escape scrutiny," reports The Guardian.

The new restrictions were implemented amid what Facebook said was part of a wider crackdown on third party plugins, including ad blockers, which harvest unauthorized data from the site.

"Ten days ago, our software stopped working, and efforts to fix it have proved much harder than before," said WhoTargetsMe co-founder Sam Jeffers, who feared his service may soon be effectively barred altogether from the social media platform. "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."

Approximately 20,000 people had voluntarily signed up with WhoTargetsMe - which Facebook has now rendered virtually useless.

"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 spokeswoman Beth Gautier in a response. "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."

At the end of 2018 - after coming under intense scrutiny from lawmakers around the world, Facebook launched its own political ad archive. While the move was lauded by campaigners, the social media giant has been slow to give journalists and researchers direct access to their database (currently available in the US, UK and Brazil, with EU implementation coming before this spring's European parliament elections).

While Facebook says its in-house transparency page is top notch, WhoTargetsMe's Jeffers says it is still "inadequate" and doesn't provide any useful information as to why a particular user is being targeted - or who is behind said advertising.