Starbucks said it took hygiene "extremely seriously." PK.Inspiration_06 / Shutterstock It's hard to match the refreshing taste of an iced coffee on a hot summer day — but a new BBC investigation may make you think twice before you order a chilled drink.

The BBC's consumer-affairs "Watchdog" program found that samples of iced drinks from Starbucks and two other UK chains — Costa Coffee and Caffe Nero — contained "varying levels of the bacteria."

Along with the ice, researchers also studied the cleanliness of tables, trays, and high chairs at 30 of the coffee-chain branches.

Costa Coffee was the worst offender, with seven out of 10 samples "found to be contaminated with bacteria found in feces."

Starbucks and Caffe Nero also tested positive for "fecal coliforms" in three out of 10 samples.

Tony Lewis, the head of policy at the Chartered Institute of Environmental Health, said the levels found were "concerning." He said the types of bacteria identified were "the source of human disease," adding: "These should not be present at any level — never mind the significant numbers found."

The BBC reported that all three chains, some of the biggest in the UK, said they had "taken action," with Costa saying it had "updated its ice-handling guidelines" and was "introducing new ice equipment storage."

Both Starbucks and Caffe Nero told the BBC they were conducting their own investigations into the claims. Starbucks said it took hygiene "extremely seriously."

You can check out the "Watchdog" program on BBC1 on Wednesday at 8 p.m. BST.