Three teenagers and a woman have been arrested as part of an investigation into cyber attacks targeting Marks & Spencer, the Co-op and Harrods.

The National Crime Agency (NCA) said the four were arrested early on Thursday on suspicion of blackmail, money laundering, offences linked to the Computer Misuse Act and participating in the activities of an organised crime group.

The arrests included a 17-year-old British man from the West Midlands, a 19-year-old Latvian man from the West Midlands, a 19-year-old British man from London and a 20-year-old British woman from Staffordshire.

All of the four were arrested at their home addresses, and had their electronic devices seized for digital forensic analysis, the NCA said.

The NCA has arrested the four on a string of suspected crimes including blackmail, money laundering and more following the attacks on retailers including M&S

They remain in custody for questioning by officers from the agency's National Cyber Crime Unit in relation to the three attacks, which took place in April this year.

Deputy Director Paul Foster, head of the NCA's National Cyber Crime Unit, said: "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.

"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.

"Cyber attacks can be hugely disruptive for businesses and I'd like to thank M&S, Co-op and Harrods for their support to our investigations.

"Hopefully this signals to future victims the importance of seeking support and engaging with law enforcement as part of the reporting process. The NCA and policing are here to help."

Just days ago, M&S chairman Archie Norman told a select committee that the cyber-attack was "traumatic" for employees from the head office to the shop floor.

"It's like an out-of-body experience," Norman told the Business & Trade sub-committee.

"It's fair to say that everybody at M&S experienced it, like our ordinary shop colleagues working in ways they hadn't worked for 30 years, working extra hours just to try and keep the show on the road.

"For a week probably the cyber team had no sleep, or three hours a night.

"It's not an overstatement to describe it as traumatic, and it has endured some weeks. We're still in the rebuild mode and will be for some time to come."