A secretive group that has spent hundreds of thousands of pounds encouraging voters to back a hard Brexit is being investigated by Britain’s data watchdog, it has emerged.

MPs had already raised concerns about the activities of the anonymous Mainstream Network group, whose ads and website encourage readers to “chuck Chequers”, referring to Theresa May’s proposed Brexit deal. It is has been estimated that it could have spent up to £250,000 on the ads.

The Information Commissioner’s Office (ICO) has now confirmed it is examining whether an email campaign by Mainstream Network, organised to lobby MPs to “bin Chequers”, may have breached new GDPR rules governing personal data.It follows the decision by the commissioner to fine the Brexit campaign group Leave.EU and an insurance company owned by Brexit-backer Arron Banks over breaches of data laws.

A report by the commissioner stated that Leave.EU and Eldon Insurance – trading as GoSkippy – were being fined £60,000 for serious breaches of the law that governs electronic marketing. It said “a disturbing disregard for voters’ personal privacy” had been uncovered.

The organisers and funders of Mainstream Network remain anonymous, while there are no details on its website. Tech experts who had examined the site also suggested that whoever set it up had gone to some lengths to hide any clues to their identity. There are no obvious records for the group held by Companies House.

As well as using ads and its website to push its anti-Chequers message, Mainstream Network has also targeted social media users with Facebook ads, encouraging them to lobby their MP to oppose May’s deal and support a total break with the EU.

Mainstream Network’s most recent stories include a report on a trip to China by Liam Fox, the international trade secretary, to champion UK exporters. In the summer, it carried an editorial that called on May to back “full Brexit or No Deal”.

Facebook Twitter Pinterest Facebook has so far declined to reveal any details it knows about Mainstream Network. Photograph: Dominic Lipinski/PA

“The biggest democratic decision this nation has ever made was done so the UK can retain control over its border, money, laws, regulations and destiny,” it states. “So any half-hearted in-out deal would be the worst of any world: we would be in the EU in all but name, without the right to set our own laws or regulations, and unable to influence the 27 other member states.” Clicking on one of the ads calls up a link to the user’s constituency and MP’s name. Another click launches a pre-written email addressed to their MP, with the Mainstream Network campaign copied in.

According to evidence gathered by the digital campaign group 89up, Mainstream Network ads could have reached as many as 10 million users. Its evidence was published by the Commons Digital, Culture, Media & Sport (DCMS) select committee, as part of its inquiry into “fake news”.

The only means of contacting Mainstream Network is through its Facebook and Twitter accounts. The Observer received no response to a request for comment yesterday. The campaign’s website continues to publish news stories supportive of Brexit. The first piece was published in February this year.

Facebook has so far declined to reveal any details about what it knows about the group. It has recently tightened rules around political ads. However, it is delaying plans to require British political advertisers to verify their identity. Damian Collins, the chair of the DCMS committee, has said Mainstream Network was a “clearly sophisticated organisation spending lots of money on a political campaign”.

Mike Harris, the chief executive of the 89up agency that revealed details of the group, said: “Even after our report to parliament’s DCMS select committee, raising concerns Mainstream Network may have been acting illegally, their Facebook page is still online and the website continues to be updated with no further information on who is behind this site. We will be sending our research to the ICO and hope they reveal the people or groups involved, the country where the advertising originated, and outline if the law was breached.”

An ICO spokesperson said: “We are aware of these reports and are investigating the circumstances of the ads to understand whether any data protection rules have been breached.”