Investigation reveals more files were illegally accessed than first thought.

3UK reminded customers to be vigilant this week, after it emerged that more files were accessed during last year’s data breach than first thought.

The mobile operator announced in November that information about 133,827 subscribers had been illegally obtained by criminals attempting to fraudulently acquire new handsets.

"During the course of the investigation, additional files were recovered as part of the same activity," 3UK said in a statement on Monday.

Details including names, addresses, mobile numbers, birthdays, handset type, contract start and end dates, upgrade eligibility dates, and payment methods were contained in these files.

"We have contacted a further 76,373 customers to advise them of the new information and apologise for the inconvenience and concern this may cause," 3UK said. "We ask customers to be cautious about anyone contacting them."

3UK reiterated that no financial information, bank details, passwords or PIN numbers were obtained by the hackers.

"No fraudulent activity has been identified against the customers we have contacted today," the operator added.

Following last year’s data breach, eight customers were unlawfully upgraded to new handsets, which the criminals intended to intercept and sell on. Three people have been arrested in connection with the breach.