A 15-year-old boy has been arrested over the TalkTalk cyber hack that compromised the data of four million customers.
A ransom demand was received by TalkTalk on Friday evening after which the firm downplayed the seriousness of the attack as it did not believe those behind it would be able to steal money using the data.
Experts have said the attack appeared to use well-establish hacking techniques.
The boy was apprehended by police yesterday in Northern Ireland on suspicion of offences under the Computer Misuse Act after an investigation by the cyber-crime unit of the London Metropolitan Police.
"He has been taken into custody at a County Antrim police station where he will later be interviewed," it said in a statement, adding police officers were searching an address in Northern Ireland in connection with the arrest.
They later said he had been released on bail pending further enquiries.
A spokeswoman for TalkTalk said the company had been informed of the arrest.
"We are grateful for the swift response and hard work of the police. We will continue to assist in the ongoing investigation," she said in a statement.
TalkTalk said on Sunday it had hired defence company BAE Systems to investigate the cyber-attack, the third on the ISP’s computer systems this year.
Shares in the company fell a further 12 per cent Monday, wiping around £360m off the company's market value since the news broke after the market closed on Thursday.
The company is likely to face tough questioning from British lawmakers about the adequacy of its defences against cyber-attack.
Parliament's Culture, Media and Sport Committee has said it will conduct an inquiry on data protection, which will likely see TalkTalk executives summoned to appear.