People with weak immune systems or underlying medical conditions are at increased risk of infection from controversial fish pedicures, experts have warned.
Questions have been raised in recent months over the beauty craze which sees customers place their feet in tanks of water containing dozens of tiny Garra Rufa fish which nibble on dead skin.
While the pampering carries a "very low" risk for healthy clients, those with conditions including diabetes and psoriasis have now been advised against the indulgence.
The pedicures - which are popular in Asia - have been banned in some US states, including Florida, Texas, New Hampshire and Washington amid fears that infections could spread through open wounds.
The Health Protection Agency (HPA) has issued new guidance after a panel found fish tank water contained a number of micro-organisms and that infections could be transmitted either from fish to person (during the nibbling process), water to person (from the bacteria which can multiply in water), or person to person (via water, surrounding surfaces and the fish).
While it advised certain individuals to avoid treatment, it said any risk was "very low" for healthy clients when spas adhered to strict codes of cleanliness.
Dr Hilary Kirkbride, consultant epidemiologist at the HPA, said: "Provided that good standards of hygiene are followed by salons, members of the public are unlikely to get an infection from a fish spa pedicure, however the risk will be higher for certain people."
Garra Rufa fish - a type of toothless carp - have become increasingly popular in salons across Britain, in part because of their novelty value. Dr Paul Cosford, director of health protection services at the HPA, said the risk posed by the fish tanks could be kept to a minimum if spas followed "strict standards of cleanliness".
Anyone considering a fish pedicure can help reduce health risks by ensuring cuts or infections on the feet or legs are given time to heal before treatment. Clients are also urged to wait at least 24 hours after having a leg wax or shaving to minimise the chances of contracting an infection.
The HPA guidance has been endorsed by the Chartered Institute of Environmental Health and the Royal Environmental Health Institute of Scotland.