More than 2.7 million of Uber's UK customers and drivers were affected in its 2016 data breach, with their names, email addresses and phone number stolen from the company's servers.
"This is an approximation rather than an accurate and definitive count because sometimes the information we get through the app or our website that we use to assign a country code is not the same as the country where a person actually lives," Uber said in a statement.
"When [the breach] happened, we took immediate steps to secure the data, shut down further unauthorised access, and strengthen our data security," the company added.
However, Uber doesn't think anyone needs to take action because no evidence of fraud or account misuse has been detected. Any accounts that Ubers thinks may have been affected have been flagged for "additional fraud protection".
Uber advised anyone who's a customer of the taxi booking service should monitor their Uber account for any suspicious activity and to contact the company if there appears to be any suspicious activity.
This advice may not go far enough to satisfy the UK's national data protection regulator, the Information Commissioner's Office (ICO).
In a statement, deputy commisioner of the ICO, James Dipple-Johnstone, said: On its own [the stolen] information is unlikely to pose a direct threat to citizens. However, its use may make other scams, such as bogus emails or calls appear more credible. People should continue to be vigilant and follow the advice from the NCSC.
"As part of our investigation we are still waiting for technical reports which should give full confirmation of the figures and the type of personal data that has been compromised.
"We would expect Uber to alert all those affected in the UK as soon as possible."
Approximately 57 million Uber users and drivers around the world were affected by the hack on its systems, which was covered up for more than a year.
"This latest shocking development about Uber will alarm millions of Londoners whose personal data could have been stolen by criminals," said London Mayor, Sadiq Khan. "Uber needs to urgently confirm which of their customers are affected, what is being done to ensure these customers don't suffer adversely, and what action is being taken to prevent this happening again in the future.
"The public will want to know how there could be this catastrophic breach of personal data security."
