Trendy fish pedicures could spread HIV and hepatitis C, officials warned.
The Health Protection Agency said risks from the foot-nibbling treatment are "low but could not be completely excluded".
And it said those with diabetes, psoriasis or with weakened immune systems are especially vulnerable and should never undergo the pampering craze.
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 like HIV and hepatitis could be transmitted if infected clients bleed in spa water that is used again.
A report added that the risk is "extremely low, however, this cannot be completely excluded".
An agency spokesman said last night: "We have issued this guidance because there are a growing number of these spas.
"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 have banned fish pedicures.
Conventional sterilisation 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 water is changed after each client.
They should also check customers for health conditions making them vulnerable to infection, and for cuts and grazes.
Hundreds of high street beauty salons, malls, hairdressers and fashion shops offer the treat.
Last night Christina Wright, boss of fish spa chain Appyfeet, accused officials of "scare-mongering". She added: "We worked for 18 months with the Health and Safety Executive and local authorities making sure our spas were of the highest standard."
A spokeswoman for HIV charity the Terrence Higgins Trust said: "The risk of picking up infections is minimal but people must be careful where they choose to go."