Fish pedicures could spread HIV and hepatitis C, officials from Britain's Health Protection Agency warned.
People with diabetes, psoriasis or weakened immune systems are especially vulnerable and should never undergo the pampering craze, the officials said.
Infections and bacteria may be passed on by the tiny garra rufa fish themselves or through water used by a previous client and left unchanged. Blood-borne viruses such as HIV and hepatitis also could be transmitted if infected clients bleed in spa water that is used again.
A report said that the risk from the foot-nibbling treatment was "extremely low, however, this cannot be completely excluded."
An agency spokesman said, "When the correct hygiene procedures are followed, the risk of infection is very low. However, there is still a risk of transmission of a number of infections -- this does include viruses like HIV and hepatitis."
Some parts of the US and Canada already banned fish pedicures. Conventional sterilization of equipment cannot take place because it would harm the fish.
In its report after a six-month review, the agency said salons must follow "strict standards of cleanliness" and ensure that water is changed after each client. They also should check customers for health conditions making them vulnerable to infection and for cuts and grazes.