The warning comes as ICO issued a fine of £4,400,000 to Interserve Group Ltd, a Berkshire-based construction company, for failing to keep the personal information of its staff secure. This is a breach of data protection law.

ICO found that the company failed to put appropriate security measures in place to prevent the cyber attack, which enabled hackers to access the personal data of up to 113,000 employees through a phishing email.

The compromised data included personal information such as contact details, national insurance numbers, and bank account details, as well as special category data including ethnic origin, religion, details of any disabilities, sexual orientation, and health information.

John Edwards, UK Information Commissioner, said: “The biggest cyber risk businesses face is not from hackers outside of their company, but from complacency within their company. If your business doesn’t regularly monitor for suspicious activity in its systems and fails to act on warnings or doesn’t update software and fails to provide training to staff, you can expect a similar fine from my office.

“Leaving the door open to cyber attackers is never acceptable, especially when dealing with people’s most sensitive information. This data breach had the potential to cause real harm to Interserve’s staff, as it left them vulnerable to the possibility of identity theft and financial fraud.

“Cyber attacks are a global concern, and businesses around the world need to take steps to guard against complacency. The ICO and NCSC already work together to offer advice and support to businesses, and this week I will be meeting with regulators from around the world, to work towards consistent international cyber guidance so that people’s data is protected wherever a company is based.”

Mr Edwards will be attending the 44th Global Privacy Assembly (GPA) in Turkey this week, where more than 120 data protection and privacy authorities will meet. At the GPA, the ICO will present a resolution calling for further international collaboration to increase cyber resilience across the world.