London (CNN Business) Tens of thousands of political ads went missing from Facebook's archive this week, according to researchers, just days before voters go to the polls in the most important UK election for decades.

Facebook FB A spokesperson forconfirmed its library went down but could not say how many political ads disappeared. The problem affected several countries, and Facebook prioritized fixing the UK database because of the imminent election.

"We have fixed the bug and all of the impacted ads in the UK are now back in the Ads Library," the spokesperson said.

Facebook did not comment on what caused the problem, which meant users and researchers lost sight of who was spending what in the run-up to polling day on Thursday.

The company launched the library in October 2018 to make it easier to see who is buying advertisements on the platform, how much they are spending and who they are targeting. Facebook and other platforms came under intense scrutiny after they were flooded by misinformation and anonymous ad spending during the US elections and Brexit referendum in 2016. Facebook was also fined for failing to protect user data in the Cambridge Analytica scandal