Facebook promised transparency on political ads. Its system crashed days before the UK election

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.

A spokesperson for Facebook (FB) confirmed 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.

UK voters will elect their next parliament on Thursday in one of the most consequential elections in modern British political history. The outcome could determine the course of Brexit, transform the British economy and raise new questions about Scotland's future in the United Kingdom.

The campaign has already been marred by a significant amount of campaign trickery and online misinformation, much of it peddled by Prime Minister Boris Johnson's Conservative Party.

Researchers for Who Targets Me, a non-profit organization that tracks online ads, flagged the problem to Facebook early on Tuesday, co-founder Sam Jeffers told CNN Business. The disappearances were first reported by Sky News.

Jeffers' team noticed that ads they had previously used in their research were no longer showing up in searches of the ad library, though they were still discoverable by using specific ID numbers. They estimated tens of thousands of ads went missing, just within the United Kingdom.

Jeffers said spending figures were also missing.

"Yesterday, if you looked at the ad library, it said Boris Johnson had spent less than £200 during the entire election, but it's more around £100,000," he said.

Twitter's decision in October to stop accepting political ads has increased the pressure on other platforms to do more to prevent abuse. Facebook's approach is controversial because it allows politicians to fun false ads. Instead of banning or fact checking ads, it has focused on initiatives such as the library to provide more transparency.

In March, Facebook made the ad library more easily searchable. People can view the top searches of the week and see a map of spending by region. The site also lets people search by name or topic, and now provides more information around where pages are managed and who controls them.

Jeffers still isn't convinced the problem has been entirely fixed. He said the daily ad spending reports Facebook had been putting out haven't been updated since December 5.

"When it breaks two days before an election ... it makes all of the other [transparency promises] ring a bit hollow," Jeffers said. "This ad library is one of their basic commitments to electoral transparency and now it hasn't worked."