Foreign hackers have penetrated some firms for up to two years and business secrets are being stolen on an "industrial scale," Government Communications Headquarters Director Ian Lobban told the BBC.
"People are going after intellectual property and then seeking to translate it into national gain," he said.
"We started a couple of years ago thinking this was going to be very much about the defense sector but really it's any intellectual property that can be harvested."
Michael Rake, chairman of BT and president of the business lobby group CBI, said he has been warning British businesses about the dangers.
"These threats are real, they're sophisticated, they do financial and reputational damage," he said.