The government will next week confirm the launch date for a UK-wide age block on online pornography as privacy campaigners continue to raise concerns about how websites and age verification companies will use the data they collect.

The plan for implementing the long-delayed age block, which has been beset by technical difficulties, is expected to be announced alongside the government’s other proposals for tackling online content harmful to children, although it could be several months before the system is fully up and running.

The age block will require commercial pornography sites to show that they are taking sufficient steps to verify their users are over 18, such as by uploading a passport or driving licence or by visiting a newsagent to buy a pass only available to adults. Websites which fail to comply risk substantial fines or having their websites banned by all British internet service providers.

Jim Killock, the executive director of the Open Rights Group, said he remained concerned about the prospect of a major data leak as a result of people handing over their personal identification: “It might lead to people being outed. It could also be you’re a teacher with an unusual sexual preference and your pupils get to know that as a result of a leak. It won’t get you sacked for viewing something legal but it could destroy your reputation.”

“Politicians don’t understand that data about their porn preferences might end up in the hands of journalists or others.”

Killock, whose organisation campaigns against state intervention online, said he was particularly worried about the role played by a single company called MindGeek, which owns many major pornography sites such as PornHub, and has founded its own age verification company called AgeID: “The problem is you’re giving all your data to the pornographic equivalent of Mark Zuckerberg: ‘This is what I like, this is who I am, and these are all of the sites I’ve visited’.”

AgeID has previously said it believes there is a market of up to 25 millions Britons for its age verification system, suggesting it believes around half of British adults will want to access online pornography through its service.

Its system will require individuals to create an account with their email and password and then upload a passport or driving licence, which will be verified by a third party. If they do not feel comfortable doing this, they can present themselves in person with appropriate ID at a newsagent to buy a so-called “porn pass” for £4.99 per device, with the owner of the shop verifying the age of the purchaser.

James Clark, the director of communications at AgeID, said its method of storing the login and password of verified users meant that “at no point does AgeID have a database of email addresses”, citing external audits of his company’s processes.

“AgeID does not store any personal data input by users during the age verification process, such as name, address, phone number, date of birth. As we do not collect such data, it cannot be leaked, marketed to, or used in any way.”

He claimed that while AgeID could not be used to link viewing data to an individual’s identity, rival age verification companies “may not be so robust” and could be prone to leaks.

The decision to impose strict content restrictions on pornography is being watched closely by parts of the mainstream British media industry, as it will set a precedent for future gatekeeping of the internet.

Despite this, the issue has received relatively little press attention and polling by YouGov seen by the Guardian shows three-quarters of Britons have not heard of the impending ban. Once they were informed of its aims, 67% said they supported the effort to require age verification, amid concerns about the impact of pornography on British teenagers.

However, just a third of Britons believe it will be effective in stopping children accessing such material, given individuals who wish to circumvent the age check will be able to do so easily using VPN services, which make a computer appear as though it is located in another country.

Social media sites such as Twitter, which contain large amounts of pornographic content, will be unaffected by the new rules.

The legislation to introduce the age restriction was approved by parliament two years ago but the implementation date has been repeatedly delayed. Despite speculation the ban could be implemented on 1 April, those with knowledge of the process say there is no chance of that date being met.

The British Board of Film Classification has been given the role of regulating the UK age verification industry, with its guidelines requiring providers to ensure they are not using consumer data without their consent.

Craig Lapper, the head of compliance at the BBFC, said the move challenged “the old certainties and attitudes that developed in the early days of the internet about ‘this can’t be regulated and it’s a free-for-all’.”

He added: “Attitudes publicly and politically have changed that nothing can, or should, be done. Because everyone is on the internet all the time, people do expect to see similar standards online.”

Other websites, notably Tumblr, have moved to restrict the availability of legal pornography on their services in recent months. The UK’s age block has already sparked one major change in the implementation of British law, with the Crown Prosecution Service redrawing its definition of what counts as legally obscene material.