Britain's highest court on Wednesday blocked a $4-billion class action against Google that had accused the company of illegally tracking millions of iPhone users.

The Supreme Court said in a statement that its five judges "unanimously dismisses" the legal action brought by campaigners against the US-based tech giant on behalf of 4.4 million people in England and Wales.

The "Google You Owe Us" association had sought compensation of £3 billion ($4 billion, 3.5 billion euros) after accusing the group of secretly tracking iPhone users' internet activity.

Following its victory, the Internet search giant insisted it remained committed to online safety.

"People want to know that they are safe and secure online, which is why for years we've focused on building products and infrastructure that respect and protect people's privacy," Google said in a statement.

The "Google You Owe Us" association described the verdict as a "bitter blow" to UK consumers.

A UK court had already dismissed the case in October 2018 but the judgement was overturned by the Court of Appeal, allowing the latest hearing to take place.

Judge George Leggatt, delivering the Supreme Court ruling, declared the intention to seek damages without proving financial loss or mental distress, was "unsustainable".

Lead claimant Richard Lloyd said in a statement that the association was "bitterly disappointed that the Supreme Court has failed to do enough to protect the public from Google and other big tech firms who break the law". 

It had accused Google of circumventing iPhone security options and collecting personal data between August 2011 and February 2012 using the smartphone's Safari browser.

It claimed Google "illegally misused the data of millions of iPhone users" via the "clandestine tracking and collation" of information about internet usage on iPhones.

The Supreme Court concluded, however, that the "unlawful processing" of data had not been proven.