A fake advert purporting to have been paid for by scandal-hit Cambridge Analytica has been posted to Facebook.
The ad, now removed but still visible in Facebook's new ad transparency portal, uses the logo of EU referendum campaign group BeLeave, which, in July, was found to have broken electoral law.
It calls for a new immigration policy, saying: "Our current system is broken."
The BBC understands it was created by news site Business Insider, which has been contacted by the BBC for comment.
Cambridge Analytica closed down operations in May.
"This ad was not created by Cambridge Analytica," a Facebook spokesman said.
"It is fake, violates our policies and has been taken down.
"We believe people on Facebook should know who is behind the political ads they're seeing, which is why we are creating the Ads Library so that you can see who is accountable for any political ad.
"We have tools for anyone to report suspicious activity such as this."
The ad was spotted by Guardian and Observer journalist Carole Cadwalladr.
"It just highlights how completely useless Facebook's new transparency rules are," she told the BBC.
"I can't think how they ever thought they were going to work - anybody can write anything in that box."
From 16 October, Facebook's new rules for political ads in the UK came into effect.
The social network had claimed that creators of such ads would have to prove their identity and location.
However, the resilience of Facebook's system has been called into question.
News site Vice was able to post ads purporting to be paid for by every sitting US senator, a total of 100 individuals.
"Receiving approval to run an ad 'paid for' by a senator typically just took a few minutes," said Vice journalist William Turton.