United Kingdom investigators also questioned whether Facebook failed to maintain adequate safeguards to ensure other third-party app developers had not misused social data.
In a statement issued in advance, information commissioner Elizabeth Denham warned that voters' faith in the political system was being eroded.
Wednesday's ICO report said: "The ICO's investigation concluded that Facebook contravened the law by failing to safeguard people's information".
Facebook founder Mark Zuckerberg earlier this year admitted his company made mistakes in a "breach of trust" with users and he ordered reforms to prevent a repeat and to give people more control over how their data is used.
Facebook is to be hit with the maximum £500,000 ($663,000) fine over data protection breaches related to the Cambridge Analytica scandal.
Because the breaches in the scope of the ICO's inquiry occurred in 2013-2014, Facebook escaped a fine of up to 4 per cent of turnover, equivalent to more than £1.2 billion, that could have applied under the general data protection regulation (GDPR), which came into force on...
Analysts expect Facebook to make $22.6 billion in profit this year, according to FactSet.
"In the new regime, they would face a much higher fine", Denham said. A group of industry executives met with France's President Emmanuel Macron to discuss how to use technology to improve people's lives.
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Denham also called for the government to introduce a statutory code of practice for the use of personal data in political campaigns, adding that "this can not be at the expense of transparency, fairness and compliance with the law".
EU Justice Commissioner Vera Jourova welcomed the ICO report.
"We are at a crossroads".
That's why greater and genuine transparency about the use of data analytics is vital'. Facebook also received a minor fine of $164,000 from French regulators for failing to meet the country's data protection rules.
On Tuesday, U.K. watchdogs announced a $664,000 preliminary fine - the maximum amount allowed - after finding Facebook lacked strong privacy protections and overlooked critical warning signs that might have prevented Cambridge Analytica from trying to manipulate public opinion on behalf of clients around the world, including those who sought to withdraw Britain from the European Union in 2016. But the ICO said that because of the timing of the incidents involved in its probe, the penalties were limited to those available under previous legislation.
The commissioner's office also said it could now bring criminal charges against the parent company of Cambridge Analytica.
International Monetary Fund investment manager Nathan Landis told The Australian newspaper most awards for privacy breaches ranged between A$1,000 and A$10,000 (US$750-$7,500).