A number of online services, such as news publishers, are asking users to either pay up or consent to being tracked while using the platform for free.
The Guardian became the latest UK publisher to start doing this in March, external, following in the footsteps of the Daily Mirror, the Independent and others.
The advertising model, known as "consent or pay", has become increasingly popular.
Publishers say they have no choice due to growing pressures on their finances, while campaigners say it is unfair to users to ask them to pay up in exchange for not being targeted with ads.
And "consent or pay" models may fall foul of data protection law in the UK if a user's consent has not been "freely given", according to the ICO.
"If organisations choose to adopt a 'consent or pay' model, they must demonstrate the model is compliant with UK data protection law," a spokesperson told the BBC.
"This means that, among other data protection standards, organisations must demonstrate that people can freely give their consent to processing of their personal information for personalised advertising."