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 will emphasize that 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.