Christopher Wylie has warned that social media companies have the capacity to artificially amplify "false and manipulative information on a wide scale".
He explained: "In most Western democracies, you do have the freedom of speech. But freedom of speech is not an entitlement to reach. You are free to say what you want, within the confines of hate speech, libel law and so on. But you are not entitled to have your voice artificially amplified by technology.
"These platforms are not neutral environments. Algorithms make decisions about what people see or do not see."
Wylie has been particularly concerned by the spread of disinformation amid the coronavirus pandemic.
He told the BBC: "There are some objectively disprovable things spreading quite rapidly on Facebook right now. For example, that Covid does not exist and that the vaccine is actually to control the minds of people. These are all things that are manifestly untrue, and you can prove that.
"Our democratic institutions and public discourse are underpinned by an assumption that we can at least agree on things that are true.
"Our debates may be about how we respond or what values we apply to a particular problem, but we at least have a common understanding that there are certain things that are manifestly true."