Facebook has appealed against a fine imposed on it by the UK's data watchdog after the Cambridge Analytica scandal.

The social network says that because the regulator found no evidence that UK users' personal data had been shared inappropriately, the £500,000 penalty was unjustified.

Last month, the watchdog said Facebook's failure to make suitable checks on apps and developers amounted to a "serious breach of the law".

It has yet to respond to the appeal.

This was the final day the US firm had to challenge the Information Commissioner's ruling.

The affair stems from the discovery that an academic at the University of Cambridge - Dr Aleksandr Kogan - used a personality quiz to harvest up to 87 million Facebook users' details.

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

It was initially reported that about 1.1 million UK-based users had had their details exposed.

But Cambridge Analytica said it had only ever licensed data belonging to about 30 million people, and a probe by the Information Commissioner's Office found no evidence that UK citizens were among them.

Even so, the ICO imposed the maximum penalty possible on Facebook on the basis that UK members had been put at risk and the tech firm had not done enough to address this after learning of the problem.

"The ICO's investigation stemmed from concerns that UK citizens' data may have been impacted by Cambridge Analytica, yet they now have confirmed that they have found no evidence to suggest that information of Facebook users in the UK was ever shared by Dr Kogan with Cambridge Analytica, or used by its affiliates in the Brexit referendum," said a statement from Facebook's lawyer Anna Benckert.

"Therefore, the core of the ICO's argument no longer relates to the events involving Cambridge Analytica. Instead, their reasoning challenges some of the basic principles of how people should be allowed to share information online, with implications which go far beyond just Facebook, which is why we have chosen to appeal.

"For example, under the ICO's theory people should not be allowed to forward an email or message without having agreement from each person on the original thread.

"These are things done by millions of people every day on services across the internet, which is why we believe the ICO's decision raises important questions of principle for everyone online which should be considered by an impartial court based on all the relevant evidence."

The appeal risks dragging out an affair that has undermined the public and politicians' trust in Facebook.

However, the BBC understands that the US firm was concerned that the ICO's ruling would form the basis for decisions taken by other regulators, which could prove more damaging still.

The ICO has itself noted that the £500,000 fine would be "significantly higher" had the EU's General Data Protection Regulation been in force earlier.

The move has not, however, changed chief executive Mark Zuckerberg's mind about rejecting an invitation to be cross-examined by MPs.