A 16-year-old boy from Norwich in eastern England was in custody and his address was being searched, London police said in a statement on Tuesday.
On Saturday, a 20-year-old man from Staffordshire, central England, was arrested, following the arrests of a 16-year-old boy in west London and a 15-year-old boy in Northern Ireland. All three have been released on bail.
TalkTalk shocked customers last month when the broadband, TV, mobile and fixed-line service provider said it had been hacked, potentially putting the private details of its 4 million customers into the hands of criminals.
The company has since said the attack was smaller than first thought. It said fewer than 21,000 unique bank account numbers and sort codes had been accessed.
Fewer than 28,000 obscured bank details were accessed, and fewer than 1.2 million customer email addresses, names and phone numbers accessed.
TalkTalk said credit and debit card details could not be used for financial transactions.
"Today we can confirm that the scale of attack was much smaller than we originally suspected, but this does not take away from how seriously we take what has happened and our investigation is still on going," Chief Executive Dido Harding said.
"On behalf of everyone at TalkTalk, I would like to apologise to all our customers."
Edited by Gadgets 360 staff from original story by Reuters
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