Up to 44 million UK Facebook users could share £2.3bn in damages, according to a competition expert intending to sue parent company Meta.
Dr Liza Lovdahl Gormsen alleges Meta "abused its market dominance" to set an "unfair price" for free use of Facebook - UK users' personal data.
She intends to bring the case to the Competition Appeal Tribunal.
A Meta representative said users had "meaningful control" of what information they shared.
Facebook "abused its market dominance to impose unfair terms and conditions on ordinary Britons, giving it the power to exploit their personal data", Dr Lovdahl Gormsen says.
And this data, harvested between 2015 and 2019, provided a highly detailed picture of their internet use, helping the company make "excessive profits".
Anyone living in the UK who used Facebook at least once during the period will be part of the claim unless they choose to opt out, she says.
However, in November, the UK's Supreme Court rejected an optout claim seeking billions of pounds in damages from Google over alleged illegal tracking of millions of iPhones - Google said the issue had been addressed a decade ago.
The judge in that case said the claimant had failed to prove damage had been caused to each individual by the data collection.
But he did not rule out the possibility of future mass-action cases if damages could be calculated.
And Dr Lovdahl Gormsen told BBC News: "Optout cases are specifically permitted at the Competition Appeal Tribunal.
"As a result, my case is able to claim damages on behalf of the 44 million British Facebook users affected."
Meta has rejected the allegations.
People use its free services because they find them useful and have control over how their data us used, it says.
A representative told BBC News: "People access our service for free.
"They choose our services because we deliver value for them and they have meaningful control of what information they share on Meta's platforms and who with.
"We have invested heavily to create tools that allow them to do so."
However, this latest case adds to the company's legal battles
The US Federal Trade Commission was recently given the go-ahead to take Meta to court over anti-trust rules.
Meta said it was sure it would prevail in court.