(Recasts with comment from Facebook and interview with researcher)
Dec 10 (Reuters) - Thousands of UK political ads went missing from Facebook's searchable advertising database on Tuesday, hampering researchers' ability to track political ads two days before Britain's general election.
Facebook's ad library, which it launched in the UK in 2018, is a public tool for users to track political advertising on the platform.
The problem, first reported by Sky News, affected ads from the ruling Conservative Party, the Brexit Party and the Liberal Democrats. It was not immediately clear if any Labour Party ads were affected.
Facebook declined to answer what caused the issue or when it would be resolved.
"Were aware that people are having trouble accessing the ads in the ads library, and were working to fix the issue as soon as possible," a Facebook spokesman said in a statement.
Facebook's ad library has been a cornerstone of the social media giant's efforts to be more transparent about its role in elections, but researchers say it is poorly maintained and fails to provide useful targeting data - for example, geotargeting information in the UK.
Tristan Hotham, a research consultant with WhoTargetsMe, an advocacy group that tracks digital political advertising, called the ads' effective disappearance "a catastrophic failure."
"If you're a researcher you expect to be able to go back and check the data. You don't expect the library to suddenly lose half of its books," Hotham told Reuters in a phone interview.
Certain ads appeared to be searchable by the ID number of the original ad, but not by the usual methods of searching for names or keywords, Reuters found.
In one example, an ad run from Prime Minister Boris Johnson's Facebook page on Dec. 4 asking voters to "end the chaos, respect the referendum and get it done" could only be located in the database by searching a 15-digit ID number.
Social media companies have been sharing more information about political advertising after U.S. intelligence agencies found that Russia targeted American voters with social media content, including ads, to try and influence the 2016 election. Russia has denied the allegations.
In October, London-based privacy advocacy group Privacy International issued a report that said Facebook, Twitter and Alphabet's Google still fail to provide adequate transparency for global users on the issue.
Facebook has also come under fire in recent months over its decision not to fact-check political ads. CEO Mark Zuckerberg has defended this stance by arguing Facebook does not want to stifle free speech, but he has also said that the company will look how the policy could be refined. (Reporting by Elizabeth Culliford in San Francisco Editing by Sonya Hepinstall)