Groups that oppose Britain's departure from the EU have outspent those in favor of Brexit on Facebook political ads by more than a third since October, according to a review of the social network's data by POLITICO.

Spending by the main anti-Brexit organizations, People's Vote UK and Best for Britain, has totalled more than £750,000 since Facebook began publishing records of political advertising on its platform, based on POLITICO's analysis.

That compares to less than £500,000 from pro-Brexit groups like Britain's Future, an organization that has been criticized for not revealing its funders, and Nigel Farage's recently launched Brexit Party, which has earmarked almost £20,000 for Facebook ads so far this month.

The ongoing digital battle comes as the British government continues its negotiations with the opposition Labour Party over a potential Brexit agreement that would allow the country to leave the European Union by the end of October.

Analysts say both pro- and anti-Brexit groups are also likely to increase their spending on Facebook ahead of the European Parliament election, which could be a test of sentiment about leaving the Union.

“You can’t leave it to an individual company to put in place what needs to be more robust, effective transparency tools" — Elizabeth Denham, U.K. privacy regulator

“Candidates and political groups need Facebook to reach the public,” said Liz Carolan, a social media expert who has analyzed the digital campaigns of several recent European elections and referendums.

Amid the ongoing uncertainty, Facebook has become the go-to digital resource for politicians and campaign groups looking to sway potential voters to their side. This mirrors similar efforts during the Brexit referendum in 2016 when groups, particularly those in favor of Brexit, used the world's largest social network to reach voters through micro-targeted posts that relied on collecting people's online data to pinpoint them with political messages.

Such use of Facebook, though, has raised concerns with policymakers who worry that politicians are increasingly reliant on the privately owned platform.

The head of the U.K.'s data protection agency criticized Facebook's relatively new political ad transparency tools, saying it should not be up to a company to offer such a service.

“You can’t leave it to an individual company to put in place what needs to be more robust, effective transparency tools," Elizabeth Denham, the U.K.'s privacy regulator, told local lawmakers this week.

POLITICO’s analysis was based on publicly available Facebook data for the period October 2018 to April 27, 2019, the latest figures available, including the spend and online reach of each Facebook ad by both pro- and anti-Brexit political and campaigning groups. The research was conducted with data support from Laura Edelson of the Online Political Ads Transparency Project at New York University.

Because of how Facebook provides the data, the figures are estimates of total spends, and may not include non-disclosed political advertising by groups that have tried to circumvent the company's push for greater transparency over how political groups use its platform.

The figures also do not include more than £250,000 of pro-Brexit ads bought by Mainstream Network, a now-defunct website that ran political messaging on Facebook against the proposed Brexit deal for at least 10 months through October 2018, according to a report for the British parliament. The company, which is now under investigation by the U.K. privacy regulator, is currently not running any ads on Facebook, based on the social network's data.

In recent months, both sides of the hotly contested argument have again turned to Facebook to target individuals. Both pro-Brexit Britain's Future and anti-Brexit People's Vote UK have purchased thousands of ad slots, often spending less than £100 each to target specific voter demographics in individual constituencies, according to the analysis.

Britain's Future, for instance, has spent more than £420,000 since October on over 4,300 Facebook ads, often sending messages to specific voters in constituents where it wanted to influence individual politicians. The pro-Brexit group, whose funding remains a mystery despite criticism over its lack of transparency, is not currently running any political ads on the social network.

People's Vote UK has used similar micro-targeting tactics, and is the largest political advertising spender on Facebook after splurging more than £430,000 on roughly 3,500 ads on the digital platform over the last seven months.

The Brexit Party, which is leading the U.K. polls ahead of next month's European Parliament election, is currently the largest weekly spender on Facebook political ads.

Its strategy has focused on criticizing high-profile Brexiteer politicians like Boris Johnson and Jacob Rees-Mogg, as well as highlighting how the country's departure from the EU may hamper the U.K.'s National Health Service. Another anti-Brexit group, Best for Britain, is third on Facebook's U.K. ad-spender list, spending around £317,000 on 890 adverts over the last seven months.

"Many of these ads are aimed at soft Brexit Tory constituencies," said Sam Jeffers, co-founder of WhoTargetsMe, a British nonprofit organization that relies on people downloading software onto their computers so the group can track political advertising on individuals' private Facebook pages.

The government joins in

Facebook political ad spending has not been limited to campaigning groups.

The U.K. government, which has vocally criticized the social network for allowing hate speech and online bullying to run rife on its platform, is at fourth place, with around £245,000 going on just 32 adverts since October. All are somewhat Brexit-related, including a series of videos in December explaining Prime Minister Theresa May's Brexit deal. But the majority of the paid-for messages are more informational than political, offering advice on passport renewal or changes to documentation needed by truckers.

Some political groups have opted for a more broad-brush approach to advertising.

The Brexit Party, which is leading the U.K. polls ahead of next month's European Parliament election, is currently the largest weekly spender on Facebook political ads. It has spent almost £20,000 on just 25 ads since early April. More than half of that spend was earmarked for just one advert aimed at raising awareness for a rally in Birmingham on April 14 at which Farage appeared.

And despite a slow start since launching its social networking page last month, Change UK, the anti-Brexit party founded by breakaway MPs from both the Tories and Labour which launched its EU election campaign Tuesday, is now the second largest weekly spender on Facebook ads, earmarking just over £15,000 on 15 ads, almost all of which were bought over the last seven days.

The three main parties — despite their criticism of Facebook's role in spreading online harms — are also big spenders, with the Conservatives, Labour and Liberal Democrats spending around £115,000, £80,000 and £90,000 respectively. Much of this advertising is not Brexit-related, and instead covers other issues like tax, the NHS, local services, police cuts, housing and how to sign up to vote.

Between late November and early 2019, the Conservatives made a concerted advertising push to encourage support for May's Brexit deal. That coincided with a period during which the prime minister toured the U.K. to advocate for the agreement ahead of a vote in parliament, which she ultimately postponed until late January.

The party has not bought any Brexit-related advertising on the social media site since December 29.

This article has been updated to reflect the latest spending amounts.