Facebook has removed threatening messages posted in reaction to a paid-for advert by a pro-Brexit group.
It said the advert itself, which attacked "Remoaner globalist scumbags" and accused pro-EU MPs of "treason", did not break its community guidelines.
But the social media giant took down posts threatening violence against anti-Brexit politicians, after being alerted to them by BBC News.
The ad, by the "Brexit Defence Force", was seen fewer than 5,000 times.
The site has vowed to do more to crack down on "hate speech" on its platforms but its team of moderators, and the artificial intelligence software it is developing, have sometimes struggled to identify inflammatory content.
The Brexit Defence Force ad was promoted to UK Facebook users on Sunday and Monday, mentioning several politicians by name and accusing them of "crossing the line of treason to our democracy".
"Make some noise on their pages, show them we are watching their treacherous devious antics and they will be held accountable," it says.
Less than £100 was spent promoting the advert but it attracted dozens of comments, some of which advocated violence.
After BBC News approached Facebook for comment, these threatening posts were deleted.
"Any content that violates our Community Standards on violence or harassment is absolutely not permitted on our platform," said a Facebook representative, adding: "This particular ad does not breach our policies."
Facebook recently unveiled new transparency measures for political adverts, following data scandals in the US and UK.
And anyone running ads in the UK now needs to verify their identity and location and carry a "paid for by" disclaimer if the ad references political figures, parties, elections, or laws.
The Brexit Defence Force Facebook page, which BBC News was able to contact through Facebook Messenger, includes this disclaimer, which references the "yellow vest" movement:
But the page gives no further information about identity or location and does not link to a website.
The Facebook representative added: "We have taken an industry-leading position on political ad transparency in the UK, introducing new tools including an archive of all political ads in a searchable ad library for seven years, going beyond what is currently required of us by law and further than anywhere else that allows political advertising."
And this "ad library" shows more than £1m has been spent on UK political ads since October - with two pro-EU groups leading the way.
Anti-Brexit groups People's Vote UK and Best for Britain, which campaign for another referendum, are responsible for about 40% of the total cash spent on UK political Facebook advertising in the past three months.
And a Telegraph investigation last week found Best For Britain adverts linking Brexit to palm oil deforestation and the death of orangutans had been viewed more than a million times.
"This is the same old scaremongering from the same people who didn't want us to leave the EU in the first place," Environment Secretary Michael Gove told the Telegraph.
"It didn't work last time and it won't work this time."
Pro-Brexit groups have spent less money on promoting their cause.
But a BBC News investigation recently revealed one of them had been buying up Google adverts in an attempt to reach the top of the rankings when people put "What is the Brexit deal?" into the search engine, putting it in an online bidding war with the government, which was trying the same tactic.
In the run-up to a cancelled Brexit vote in December, the government spent over £100,000 of taxpayers' money promoting its deal on Facebook and spent money on Google and Twitter ads.