A 15-year-old teenager has been arrested in Co Antrim in connection with the TalkTalk security breach.
Metropolitan Police said the teen was arrested at around 16:20 by PSNI officers on suspicion of Computer Misuse Act offences.
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.
"He has been taken into custody at a County Antrim police station where he will later be interviewed. 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 spokesperson said: "TalkTalk can confirm that we have been informed by the Metropolitan Police of the arrest of a suspect in connection with the cyberattack on our website on 21stOctober 2015.
"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.
North Antrim MP Ian Paisley said: "The type of attack suffered by TalkTalk once again highlights the significant danger that cybercrime poses and the impact that it can have on the community.
"Virtually every household will have banking or other personal information stored by companies on computer systems and should those systems be compromised it can leave people vulnerable to a range of crimes.
"I welcome that the PSNI have secured evidence and been able to make this arrest. I know that in this instance they are working hand in hand with the Metropolitan police as they investigate a crime which has caused concern to people right across the United Kingdom.
"I will be meeting with police again to seek updates on the progress of the investigation."
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.