The BBC reports Scotland Yard saying that a 15-year-old boy has been arrested in Northern Ireland on suspicion of Computer Misuse Act offences relating to the TalkTalk hack.
The Metropolitan Police confirmed an ongoing house search in County Antrim begun on Monday afternoon and that the boy has been taken into custody at a police station in Antrim.
An official statement says this is a joint investigation involving the Police Service of Northern Ireland (PSNI), and detectives from the Metropolitan Police Cyber Crime Unit (MPCCU).
Shares in firm fell 12 percent on Monday.
TalkTalk executives are to be summoned before MPs to explain how the hacker(s) stole data from up to four million customers; the inquiry into the hack and data protection will be launched by Jesse Norman, chair of the Culture, Media and Sport Select Committee.
Culture minister Ed Vaizey also told the House of Commons the government was not against compulsory encryption for firms holding customer data.
As police continue to investigate on their end, TalkTalk updated its data breach notification page on Sunday to say the attack didn't take place on its core systems; rather it occurred on its website. The company also further clarified what data might have been compromised.
No My Account passwords were accessed, TalkTalk wrote, and the accessed credit cards were never saved in full. Certain numbers were hidden, thereby making it impossible to use the numbers for financial transactions.
"We now expect the amount of financial information that may have been accessed to be materially lower than initially believed and would on its own not enable a criminal to take money from your account," the notification states.
More to follow.Ashley Carman, reporter at SCMagazine.com, contributed to this story.