EU data chiefs rally behind UK over Cambridge Analytica

"In case other entities are to be investigated into Facebook, the other data protection authorities will be involved in the case," he said.

The move comes ahead of growing backlash against the US social media giant and the UK-based Cambridge Analytica, a political data firm, for having used dubious methods to sway the will of people into demanding Britain leaves the European Union.

In the US, it mapped the behaviour of 50 million people to best target political campaign messages in favour of US president Donald Trump.

Of those, some consented to have their data used after taking a 2014 personality survey. Those that consented were led to be believe it was for academic purposes but the information was then handed over to Cambridge Analytica.

The European Union is set to launch a far-reaching data protection regulation at the end of May, which could impose massive fines and sanctions for firms caught breaching the rules.

"If you decide that your data is going to be used for the purposes of scientific research you have to be aware of the fact that you agreed to that, that you agreed for rules that were proposed by the researcher," said Wiewiorowski.

But he added that if the same data has then been transferred to another entity or other researchers or used for different purposes then "it might be a case of a data breach."

The probe into Cambridge Analytica will fall under the current and old EU rules dating back from 1995, upon which the UK has based its own national laws.

At stake is how data belonging to Facebook users and their associated online friends were used for purposes beyond their control.

Personal data must be respected

Both Facebook and Cambridge Analytica were also broached on Thursday by EU leaders in a summit in Brussels.

"Social networks and digital platforms need to guarantee transparent practices and full protection of citizens' privacy and personal data. EU and national legislation must be respected and enforced," EU leaders said in the conclusions of their meeting.

"We take this issue very seriously," European Council president Donald Tusk told reporters.

The issue is set to be discussed again over dinner on the eve of an EU-Western Balkan summit in Sofia in May, along with broader talks on innovation and research.

Belgium's prime minister Charles Michel also weighed in, telling reporters that the talks set for Sofia will require finding "a balance between the development of a digital market and the protection of the rule of law and democracy against potential manipulations."

Facebook CEO Mark Zuckerberg has since offered an apology given that millions of users had their private information harvested without their permission.

European Parliament president Antonio Tajani insists he appear in front of the assembly in Brussels to explain what had happened.

'A big mistake'

The parliament has no powers to subpoena the Facebook CEO but Tajani said Zuckerberg would be making "a big mistake" if he refused the invitation.

"We are the biggest market for Facebook and I think it is also important for Facebook to explain this choice in front of the European parliament in Brussels," he said.

His views were echoed by British centre-left MEP Claude Moraes, who heads the civil liberties committee, and who had the led inquiry into mass surveillance after the Edward Snowden revelations.

"I don't think the world has caught up yet of how damaging this could be both to the individuals involved but also in terms of what it means in criminal law and what it means in terms of the affects of mass manipulation," he told EUobserver.

He said greater transparency is needed to pry up the case to find out if right wing groups had also used Cambridge Analytica.

The firm's whistleblower Christopher Wylie had described it as "a full service propaganda machine". Facebook's Paul Grewal, a vice president and deputy general counsel at the social network told The Times last week it "was a scam — and a fraud".

Facebook is in a similar legal challenge from Belgium's data protection commissioner for having reused the data of friends of Facebook users, in an on-going case that was initiated almost three years ago.

This article was corrected at 12:26 on 23 March 2018. The original article had quoted Wiewiorowski as saying the "EU agreed to that" when in fact he said "you agreed to that".