Political ads are "disappearing" from Facebook's advertising monitoring system, leaving stunned researchers fearing a "catastrophic" loss of transparency and accountability.
Ads from the Conservatives, Brexit Party and Liberal Democrats, as well as local parties and candidates, no longer appear in Facebook's Ad Library, the tool set up by the tech firm to display political ads.
A search on the Ad Library for "Boris Johnson" now claims the Prime Minister's page has spent £181 since November 8. In fact, it has spent over £90,000.
It is not known how many ads have vanished, but researchers believe it could be "tens of thousands, perhaps hundreds of thousands".
As a result, it is no longer possible to find ads that have previously been reported, or perform accurate large-scale analysis using the data provided by Facebook about each ad.
"This is catastrophic," said Tristan Hotham, a researcher with ad transparency non-profit WhoTargetsMe.
"Almost all the ads from late October to early December are gone. In effect it is deleting the data."
The vanished posts include targeted ads run by the main political parties, such as a Tory campaign calling on voters to "end the gridlock" by voting Conservative.
Versions of the ad featuring opposition leaders were shown in Remain-voting constituencies St Albans, Wimbledon and Putney, whereas versions featuring Boris Johnson were shown in Leave-voting constituencies Rother Valley and Ashfield.
None of these ads now appear in Facebook's Ad Library, whether it is accessed through the public interface or the Ad Library API, which gives journalists and researchers more detailed access to the library's data.
The ads can only be found by someone who has saved the ID of the original ad. With this information, which is rarely collected, even by researchers, it is possible to locate the ad.
"Facebook must work harder to provide the tools we need to hold the powerful to account," Mr Hotham said.
"Every forward step taken seems to have strings attached."
"It is clear that Facebook must do more to uphold the values of transparency that are central to running a free and fair election."
Facebook launched the Ad Library in March 2019 response to the criticism it faced over the spread of disinformation in the 2016 EU referendum and Presidential election.
Saying it was "working hard to make advertising more transparent and to give people more information about the ads they see," the firm promised to keep ads in the library for seven years.
Despite complaints about its flawed functionality, the Ad Library has become an essential tool of journalists and researchers, who rely on it to impact of the millions of pounds spent by political parties on pushing targeted messages to voters.
A Facebook source told Sky News the company was "urgently investigating".
The Brexit Election on Sky News - the fastest results and in-depth analysis on mobile, TV and radio.
- Watch Dermot Murnaghan live from 9pm on 12 December
- See the exit poll at 10pm
- Watch [email protected] election special on 13 December
- Find out what happens next in All Out Politics special from 9am with Adam Boulton