A 15-year-old boy is in custody after being arrested in connection with the TalkTalk cyber attack.
The teenager was detained in Northern Ireland on suspicion of Computer Misuse Act offences, the Metropolitan Police said on Monday night.
He was taken into custody at a County Antrim police station to be interviewed.
A Scotland Yard spokesman said: "An arrest has been made in connection with the investigation into alleged data theft from the TalkTalk website.
"At approximately 4.20pm, officers from the Police Service of Northern Ireland (PSNI), working with detectives from the Metropolitan Police Cyber Crime Unit, executed a search warrant at an address in County Antrim, Northern Ireland.
"At the address, a 15-year-old boy was arrested on suspicion of Computer Misuse Act offences.
"A search of the address is ongoing and inquiries continue."
The investigation is being carried out by the Met's cyber crime unit, the PSNI's cyber crime centre and the National Crime Agency.
A TalkTalk spokesman said: "TalkTalk can confirm that we have been informed by the Metropolitan Police of the arrest of a suspect in connection with the cyber attack on our website on October 21.
"We know this has been a worrying time for customers and we are grateful for the swift response and hard work of the police. We will continue to assist in the ongoing investigation."
TalkTalk chief executive Dido Harding said earlier she still was unsure how many of its four million UK customers had been affected by the attack, which had affected the telecoms giant's website rather than its "core systems".
Jesse Norman, chair of the Culture, Media and Sport Select Committee, is leading an inquiry into the alleged data breach.
Officers are investigating a ransom demand sent to the the phone and broadband provider by someone claiming to be responsible and seeking payment. The firm said it was not sure if the message was genuine.
The latest breach is the third in a spate of cyber attacks affecting TalkTalk in the last eight months, with incidents in August and February resulting in customers' data being stolen.
TalkTalk saw more than a 10th of its value wiped off in trading today as shares plummeted more than 12%, adding to the 4.3% drop suffered on Friday.