A 19-year-old man has pleaded guilty to involvement in the major hack attack of the telecoms firm TalkTalk last year.
Daniel Kelley, from Llanelli, Carmarthenshire, pleaded guilty to hacking at the Old Bailey.
Judge Paul Worsley warned Kelley that, despite having no previous convictions, jail time was "inevitable" and that he should "prepare" himself.
Personal data belonging to nearly 157,000 customers was stolen from TalkTalk in October 2015.
Earlier, a 17-year-old boy, who cannot be named because of his age, was sentenced after a separate hearing at Norwich Youth Court.
Kelley admitted hacking into TalkTalk's systems to get customer data and also demanding a payment of 465 bitcoins.
That amount of the digital currency is currently worth £286,295.
He also pleaded guilty to further charges including blackmail and money laundering.
Kelley has been bailed until his sentencing, scheduled for 6 March 2017.
Email addresses and bank details were taken after the firm's website was breached, an incident that TalkTalk later said had cost it £42m.
A total of six people, all aged under 21, have been arrested as part of the police investigation into the matter.
Reacting to the prosecutions, a TalkTalk spokesman said: "The Metropolitan Police and the CPS have worked hard to find those responsible for the cyber attack, and we are pleased that their investigations have led to two successful prosecutions.
"We hope that this outcome will encourage other companies to inform their customers and the police quickly when they are attacked."
In October, the Information Commissioner's Office imposed a record fine of £400,000 on TalkTalk for poor website security.
Kelley's hacking offences involved TalkTalk and half a dozen other organisations.
One of those organisations included a Welsh further education college, Coleg Sir Gar, where Kelley was a student.
Targeting his college's website with a distributed denial of service (DDoS) attack interfered with a local hospital nearby, the court heard.
This had the effect of delaying the return of medical files to doctors.
"We are relieved that the perpetrator of these attacks has been brought to justice and wish to thank the police for their excellent support throughout this difficult period," said Mike Williams, the college's assistant principal for planning, information and finance.