Labour MP Ben Bradshaw | Christopher Furlong/Getty Images UKIP whistleblowers raise concern over Breitbart influence on Brexit vote: report UK Electoral Commission declines to pursue allegations brought forward by UKIP sources.

Two U.K. Independence Party insiders filed complaints with the U.K. Electoral Commission over fears that Breitbart, a right-wing American website, influenced the party in the months leading up to the Brexit vote, the Guardian reported Monday.

Speaking on condition of anonymity, the sources told the British paper that Breitbart had made “unusual arrangements” with UKIP ahead of the referendum on EU membership in June 2016. They alleged that the website paid senior volunteers for their work with the party and voiced concerns that the financial support amounted to a political donation by a foreign donor.

The news comes amid fears over the role that "dark money" may have played in the EU referendum, after Labour MP Ben Bradshaw urged the British government to probe into potential "foreign interference."

Several UKIP sources told the Guardian they were alarmed about what they considered may have been a "deliberate strategy" by Breitbart to push UKIP to pursue more far-right positions — including anti-migrant messages — ahead of the vote. Former UKIP leader Nigel Farage's close relationship with Steve Bannon, the website's founder and CEO from 2012 to 2017, was also cause for concern, they said.

Breitbart's U.K. edition, launched in 2014, was widely considered to be aligned with UKIP, and Farage has often credited Breitbart's "supportive voice" on Brexit as a decisive factor in the Leave camp's successful campaign.

British political campaigns are prohibited from accepting foreign donations or donations from people who are not U.K.-registered, but volunteer-work represents a "gray area," according to the Guardian.

The Electoral Commission brought the concerns to the attention of the Metropolitan police, which decided the allegations did not meet the threshold for further investigation, the Guardian reported.