The UK's data protection watchdog, the Information Commissioner's Office, has found the company lacking sufficient privacy protections and failing to catch third party companies like Cambridge Analytica misusing its users' data despite warnings.
In a statement issued in advance, information commissioner Elizabeth Denham warned that voters' faith in the political system was being eroded.
Finally, in a hugely optimistic bid, Denham has called for an "ethical pause" to allow lawmakers, regulators, political parties, online platforms and the public "to reflect on their responsibilities in the era of big data before there is a greater expansion in the use of new technologies". "But this can not be at the expense of transparency, fairness and compliance with the law".
The amount is the maximum allowed under the Data Protection Act 1998, but is pocket change for a company valued previous year at around $590bn (£445bn).
Facebook will address the proposed penalty before the watchdog makes a final ruling.
"We are fully cooperating with the investigation now under way by the Australian Privacy Commissioner and will review any additional evidence that is made available when the UK Office of the Information Commissioner releases their report", the spokeswoman said.
The Information Commissioner's Office announced the fine as it revealed it was preparing a criminal prosecution against Cambridge Analytica's parent company, SCL Elections Ltd.
British lawmakers have launched an inquiry into "fake news" and its effect on election campaigns, and have increasingly focused on Cambridge Analytica.More news: Royal Family steps out for Prince Louis' christening
Facebook has said it will be reviewing the report and responding to the ICO soon.
The ICO is also planning audits of the main credit reference companies and Cambridge University Psychometric Centre, and ordered Canadian data slurpers Aggregate IQ to stop processing data retained on United Kingdom citizens.
"Given that the ICO is saying that Facebook broke the law, it is essential that we now know which other apps that ran on their platform may have scraped data in a similar way", he said. The firm had links to Donald Trump's presidential campaign in 2016 and had been under investigation by ICO since May 2017.
The next phase of the ICO's work is expected to be concluded by the end of October.
Facebook faces several other investigations, including others in Europe, a probe by the US Federal Trade Commission and, reportedly, several others at federal agencies such as the FBI and the Securities and Exchange Commission.
"We must change this fast as no-one should win elections using illegally obtained data", she said, adding: "We will now assess what can we do at the European Union level to make political advertising more transparent and our elections more secure".
"Our day in British court may be within reach", he told Reuters in an email.
SCANDAL FALLOUT, The fines are starting to come for Facebook in several countries.