The Information Commissioner's Office (ICO) said Facebook had let a "serious breach" of the law take place.

The fine is the maximum allowed under the old data protection rules that applied before GDPR took effect in May.

The ICO said Facebook had given app developers access to people's data "without clear consent".

In July, the ICO notified the social network that it intended to issue the maximum fine.

Confirming the fine, it said in a statement: "Between 2007 and 2014, Facebook processed the personal information of users unfairly by allowing application developers access to their information without sufficiently clear and informed consent, and allowing access even if users had not downloaded the app, but were simply 'friends' with people who had."

"Facebook also failed to keep the personal information secure because it failed to make suitable checks on apps and developers using its platform."

Facebook said it was "reviewing" the ICO's decision.

"While we respectfully disagree with some of their findings, we have said before that we should have done more to investigate claims about Cambridge Analytica and taken action in 2015," it said in a statement.

What was the Cambridge Analytica data scandal?

Researcher Dr Aleksandr Kogan and his company GSR used a personality quiz to harvest the Facebook data of up to 87 million people.

Some of this data was shared with Cambridge Analytica, which used it to target political advertising in the US.

"Even after the misuse of the data was discovered in December 2015, Facebook did not do enough to ensure those who continued to hold it had taken adequate and timely remedial action, including deletion," the ICO said.

The ICO found that more than one million people in the UK had their data harvested by the personality quiz.

"A company of its size and expertise should have known better and it should have done better," said Information Commissioner Elizabeth Denham.

The ICO is still investigating how data analytics is used for political purposes.

Ms Denham is due to give evidence to the Department for Digital, Culture, Media and Sport (DCMS) Select Committee on 6 November.