An anonymous pro-Brexit Facebook ad campaign calling on users to lobby their MP to “chuck Chequers” has reached almost 11 million people in less than a year, it has emerged.

According to evidence published by Parliament’s powerful Digital, Culture, Media and Sports committee on Saturday, millions of people have been micro-targeted by a website called the ‘Mainstream Network’ in the past 10 months.

Thought to be targeting users depending on which constituency they live in, the adverts encourage viewers to tell their MP to “bin” Theresa May’s proposed Chequers Brexit deal.

When users click on the ads, a default email addressed to their MP is generated.

While the ad campaign is estimated to have cost £257,000, the site has no named organisation, UK address or group associated with it.

DCMS committee chairman Damian Collins – who is one of many MPs whose constituents have been targeted – said it was clear a “sophisticated organisation spending lots of money” was behind the campaign.

“The only people who know who is paying for these adverts is Facebook,” the Tory MP said.

The evidence – supplied to the committee by comms agency 89up as part of its inquiry into fake news – comes just days after Facebook announced a crackdown on political advertising in a bid to improve transparency.

Under the new rules, political advertisers will be required to prove their identity and will no longer be able to publish “dark ads”.

Collins said the Mainstream Network incident was an opportunity for social media giant Facebook to “show it is committed to making change happen”.

“If you are targeted with a message or asked to do lobby your MP, you should know exactly who is behind the organisation asking you to do it,” he added.

The evidence has also raised concerns that the website could be in breach of GDPR rules. 

According to 89up, when a “chuck Chequers” email template is generated by the ad, the site not only includes the MP’s email address in the message, but automatically adds its own email address to the BCC field.

This has sparked fears the website is collecting the sender’s email address for further marketing purposes.

Facebook said in a statement that all advertisers, including Mainstream Network, would need to identify themselves to continuing using its platform by 7 November.

“These steps must happen or the advertiser will be prevented from running ads related to politics on Facebook,” the company said.

HuffPost UK has contacted Mainstream Network for comment. 

The news comes a day after it was announced that the UK’s former deputy prime minister Nick Clegg is joining Facebook as the company’s head of global affairs.

The ex-Lib Dem leader – who led the party into coalition with the Conservative Party in 2010 – said in a statement on Friday he was “looking forward” to taking on the new role. His hiring comes a year after he lost his Sheffield Hallam seat in the 2017 general election.

UPDATE: This article was updated on 20 October with a comment from Facebook.