Three of the biggest fast food chains in Britain have been found to serve drinks that were contaminated.

According to a report released by investigators from BBC One's consumer series Watchdog, which will be aired at 8pm tonight (19 July), ice drinks at KFC, Burger King and McDonald's were found to contain "faecal coliform", which carry a host of diseases.

The investigators tested 10 drinks at each of the three chains and found coliforms at three McDonald's restaurants, while the number of drinks that tested positive rose to six and seven at Burger King and KFC respectively.

Laws state the acceptable level for this kind of bacteria is strictly zero, but four of the samples taken at Burger King and five of those taken at KFC were found to contain "significant" levels of contamination.

"It's extremely worrying," said Tony Lewis, head of policy and education at the Chartered Institute of Environmental Health.

"When we're finding the sorts of numbers we're finding here, you have to look at the people making the ice, handling the ice, which they then transfer into customers' drinks.

"And then you also have to look at hygiene failure with potentially the machines themselves: are they being kept clean?"

A spokesman for McDonald's reiterated the fast food giant has robust procedures in place with regard to the production, storage and handling of ice in its UK restaurants.

"Nothing is more important than the safety of our customers and people and we will continue to review our procedures and training, working closely with our restaurant teams to ensure those procedures are adhered to at all times," he said.

"Hygiene and safety practices are of the utmost importance to us and we're proud that 99% of our restaurants have an independent hygiene rating of either good or very good."

A KFC spokesman said the company was extremely disappointed and shocked by the findings, adding it too had strict procedures for the management and handling of ice, including daily and weekly inspections and cleaning of the ice machine and storage holds.

"We immediately shut down the ice machines in the restaurants affected to conduct a thorough clean and inspection, and reinforced the importance of adhering to our strict procedures to all employees," he explained.

"The restaurants all have an environmental health officer hygiene rating of four, or, five out of five, and we are awaiting the results of independent testing of the ice that will confirm they are back up to the standards we expect."

Last month, Watchdog found samples of iced drinks from Costa Coffee, Starbucks and Caffe Nero were contaminated with a faecal bacteria.

Seven out of 10 samples of Costa ice were said to be contaminated with bacteria found in faeces, known as faecal coliforms.

At both Starbucks and Caffe Nero, three out of 10 samples tested contained the bacteria.