You may want to think twice before you take a sip of your favorite iced coffee beverage this summer, especially if you're in the U.K.

The BBC's consumer-affairs Watchdog program found that ice from three of the U.K.'s biggest coffee chains — Starbucks, Costa Coffee and Caffe Nero — had been contaminated with bacteria found in feces.

Tony Lewis, head of policy at The Chartered Institute of Environmental Health, told the BBC that the level of bacteria found at these chains was "concerning" and that the types of bacteria identified were a "source of human disease."

"These should not be present at any level — never mind the significant numbers found," he said.

At Starbucks and Caffe Nero, three out of the 10 samples of ice collected were found to contain the fecal bacteria. However, at Costa seven out of the 10 samples tested positive.

"We were disappointed with the findings, especially as these stores are all rated Very Good with the top Hygiene Rating of 5," Costa told CNBC in a written statement. "It is extremely important to us that all our stores operate to high standards of hygiene at all times and we take it very seriously when any store fails to meet these standards. ... Following these results we have taken immediate action to update our ice handling procedures."

The BBC reported that both Caffe Nero and Starbucks will be conducting investigations. Neither company immediately responded to CNBC's request for comment.