A U.K. spy chief is warning that Britain and its allies risk losing a cyber conflict against adversaries like Russia unless cybersecurity is treated with greater urgency.

The head of the U.K.'s National Cyber Security Centre warns that hostile nations like Russia, Iran and China are behind the most serious cyberattacks in the U.K. In a speech Wednesday, Richard Horne said British businesses need to prepare for large-scale cyberattacks, especially if the U.K. becomes involved in international conflicts.

British retailer Marks & Spencer says it's “working day and night” to resolve a cyberattack nearly two weeks ago that has left it unable to process any online orders.