The Conservatives are at risk of breaking data laws by gathering information about pro-Brexit voters through an online campaign, it has emerged.

A Tory party webpage calling on voters to “Back Brexit” is cause for concern, according to legal advice from a leading data protection lawyer.

The page states: “MPs can’t pick and choose which votes to respect. But some are. Don’t let them get away with it.” Users are then invited to enter their name, email address and postcode.

The page suggests the data may be used to build profiles of individuals to target online advertising at them.

A legal opinion by data and privacy specialist Robin Hopkins states the party could be in breach of privacy regulations by gathering personal data, including information about voters’ political opinions and online behaviour, without giving them enough information about what data was being collected, or enough choice about how it would be used.

The legal opinion was requested by the Good Law Project, which was behind the case in the Scottish courts that saw Boris Johnson’s suspension of parliament ruled “unlawful”.

It states: “The Good Law Project is concerned the Conservative party’s current practices fail to respect the [Information Commissioner’s Office] recommendations and the requirements of data protection and privacy law. It is concerned about the risk of unlawful data practices and the potential impact of those practices on the democratic process.” Hopkins writes: “I am asked for my opinion on whether there is substance to those concerns ... my answer is yes.”

He concludes: “At least as regards the use of data for ‘online advertisements’, the individual does not seem to have been adequately informed.

“She [the user] is directed to a privacy policy that says quite a lot about how the Conservative party uses personal data – including the building of profiles about individuals and their likely voting intentions. But ... the privacy policy says nothing about how the Conservative party uses personal data to direct online advertisements at individuals based on what it knows or thinks about them.

“The individual visiting the ‘Back Brexit’ webpage is not helped to understand what this means, how it works and what she can do about it.”

The Tory party did not respond to requests for comment. Concerns over “micro-targeting” of voters with tailored, online ads emerged in the wake of the EU referendum. The Vote Leave campaign, led by Dominic Cummings, used the technique to reach users on Facebook. Cummings is now the prime minister’s chief adviser.

The Conservative Party has also said it is reviewing its Facebook advertising after it was accused of misrepresenting a BBC story. An advert featured the corporation’s logo with an article boasting of a “£14bn pound cash boost for schools”. However, a BBC story linked in the ad said the figure was actually £7.1bn.