An anonymous pro-Brexit website posing as a news organisation has been secretly encouraging voters to lobby their MPs, demanding they "bin Chequers" and "secure a full Brexit", according to evidence submitted to MPs and published on Saturday.
The website, Mainstream Network, is accused of spending hundreds of thousands of pounds on Facebook to target voters in specific constituencies and promote pro-Brexit messages, in a report by communications firm 89up given to the parliamentary fake news inquiry.
No information is given on the website or on social media about who owns, funds or writes for the website.
Damian Collins, chairman of the fake news inquiry, said: "Here we have an example of a clearly sophisticated organisation spending lots of money on a political campaign, and we have absolutely no idea who is behind it.
"The only people who know who is paying for these adverts is Facebook."
Mainstream Network did not responded to requests for comment sent to its generic email address and Facebook did not respond to requests for comment before publication.
Researchers at 89up estimate that the Mainstream Facebook page, which has 13,000 followers, has spent up to £257,000 to promote posts across the social network, potentially reaching almost 11 million people in the process.
These figures would only be reached if every share, comment or like on every post published by the Mainstream Facebook page was the result of paid advertising, however.
Mike Harris, chief executive of 89up, said: "We've seen very little evidence of significant organic reach but we know they have been running a very large set of adverts on Facebook.
"This has led us to believe the huge number of engagements is due to paid advertising."
Mainstream also has a Twitter account with paid-for tweets but with a much smaller following.
Both the website and Facebook page were also registered in late November 2017, publishing pro-Brexit news stories and opinion pieces on a daily basis and describing itself as a "News and Media Organisation".
But researchers say they found more than 70 Facebook adverts as recently as this week encouraging viewers to "tell your local MP to bin Chequers", another name for the Prime Minister's proposals for leaving the European Union.
"We have reason to believe that this is part of a sophisticated advertising campaign, with pages setup [sic] to target users in every key constituency in the UK, with bespoke localised messaging; encouraging users to lobby their respective MPs," they say in the report.
Facebook updated its policies on political advertising in the UK on Tuesday, meaning advertisers must disclose funding if they want to promote a political message.
The Mainstream Facebook adverts targeting MPs are no longer viewable but the pages on the website remain.
At least 100 MPs, including Mr Collins and defence secretary Gavin Williamson, are the subject of separate pages on the Mainstream website, each with a picture of the MP's local constituency and a button to email them.
The button opens an email on the user's computer addressed to the MP and includes the same text as the webpage, demanding the MP "tell the Prime Minister to bin the Chequers deal" and "deliver full Brexit".
Mainstream Network also adds its own email into the "blind" address field, meaning it receives a copy of the email each user sends but the MP is unaware.
"That's the point where the behaviour is illegal as they were copying themselves into the email to the MP to collect peoples' email addresses," alleged Mr Harris, suggesting that the addresses would be used for further marketing campaigns.
Data protection laws state that any organisation collecting or storing individuals' personal data, such as email addresses, must be transparent in how and why the data is collected. No such information exists on the website.
Mr Collins said: "While debate on one of the central issues facing our country is part of a thriving democracy, there is an important question of where campaigning stops and political advertising starts.
"Facebook has recently announced a set of changes to increase transparency around political advertising on its platform.
"This example offers Facebook an opportunity to show it is committed to making that change happen – if you are targeted with a message or asked to lobby your MP, you should know exactly who is behind the organisation asking you to do it."
Rob Leathern, director of product management at Facebook, said: "On November 7, all advertisers will have new requirements before they can place political ads in the UK, including Mainstream Network.
"These advertisers will need to confirm their identity and location through an authorisations process and accurately represent the organisation or person paying for the ad in a disclaimer.
"These steps must happen or the advertiser will be prevented from running ads related to politics on Facebook. We know we can't prevent election interference alone and offering more ad transparency allows journalists, researchers and other interested parties to raise important questions."