Police on Thursday arrested three teenagers and a 20-year-old woman suspected of involvement in crippling cyberattacks earlier this year against the retailers Marks & Spencer, the Co-op and Harrods.
The National Crime Agency has for months been probing the cyberattacks against the three retailers, which saw hackers demand ransom payments after breaking into their IT systems.
NCA officers detained the four on suspicion of blackmail, money laundering, offences linked to the Computer Misuse Act and participating in the activities of an organised crime group.
The teenagers, all male, included a Latvian national aged 19.
M&S said in May that some personal data of its customers had been stolen in the cyber breach that crippled its online services for weeks.
Co-op has also admitted that the hackers were able to access and extract data relating to a “significant number” of its customers from one of its systems.
"Since these attacks took place, specialist NCA cybercrime investigators have been working at pace and the investigation remains one of the agency's highest priorities," Paul Foster, head of its National Cyber Crime Unit, said.
"Today's arrests are a significant step in that investigation but our work continues, alongside partners in the UK and overseas, to ensure those responsible are identified and brought to justice," he added.
Earlier this week, M&S chairman has called for a legal requirement for businesses to report material cyberattacks to the authorities, claiming two recent major attacks on large UK firms had gone unreported.