Elizabeth Denham, the Information Commissioner, wrote in her accompanying report that Facebook should have done more to explain to its users why they were targeted for specific political advertisements or messaging. The fine "sends a clear signal that I consider this a significant issue, especially when you look at the scale and the impact of this kind of data breach".
Her office is leading the European investigations into how such an amount of data - most belonging to U.S. and United Kingdom residents, she says - could have ended up in the hands of a consulting firm that worked on Donald Trump's USA presidential campaign.
A Facebook spokeswoman said the company was cooperating fully with an investigation by Australia's privacy commissioner.
Facebook's chief privacy officer Erin Egan said in response to the ICO's report: "As we have said before, we should have done more to investigate claims about Cambridge Analytica and taken action in 2015".
The watchdog also announced plans to a bring a criminal action against Cambridge Analytica's defunct parent company SCL Elections.
"This can not by left to a secret internal investigation at Facebook".
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"Facebook should now make the results of their internal investigations known to the ICO, our committee and other relevant investigatory authorities".
"The number of Facebook users affected by this kind of data scraping may be far greater than has now been acknowledged".
Facebook broke the law by failing to safeguard people's data and not being transparent about how that data could be harvested, said Ms Denham.
The decision was welcomed by former Cambridge Analytica employee and whistleblower Christopher Wylie. "We have been working closely with the ICO in their investigation of Cambridge Analytica, just as we have with authorities in the USA and other countries".
Politicians are calling for greater transparency from Facebook in light of the ICO fine.
That's despite earlier estimates that 2.7 million users in the European Union had their data improperly shared. The region's competition chief said the social media company had provided misleading information about its privacy promises during its 2014 acquisition of the messenger app WhatsApp.
"It's an important moment for data protection", she added.