The Conservative Party may have broken the law by using "misleading" surveys and online games to hoover up voters' personal data without properly asking for consent, experts have warned.

In a series of Facebook ads, the party offered voters the chance to take a questionnaire on its website about their priorities for the NHS, and to enter their salary into an income tax calculator to find out how much money they could save under a Tory government.

Yet anyone using those tools was required to submit personal information such as their email address, their salary, their name and their postcode which the party could use to profile them or target them with social media adverts.

Users were told that this data might be used to contact them via email, but were not clearly informed of the wide range of other purposes to which it could be put and were not given the opportunity to withhold their consent.

Instead of being allowed to opt out of data collection, users were simply told that by clicking the "submit" button they would agree to the Conservative Party using their data for purposes such as "email, online advertisements and direct mail".

On Monday, after inquiries from The Telegraph, the party quietly removed the NHS questionnaire from its website, saying it "regularly reviews" and "updates" its data collection practices to comply with the law.