Although users are told that the lenses can be worn continuously for up to 14 days, studies have shown that they are not as sterile as first believed.

Now a new study has underlined the risks of ulcers. Eleven wearers, with an average age of 26 and who had worn their lenses continuously for between two and 12 days, had 13 corneal ulcers.

They sought medical care between one and four days, and all had used their lenses correctly.

They were all treated with quinolones, and vision returned to normal in every case.

Researchers reckon that ulceration may occur in about one in 5,000 extended wear lens users, which is about 10 times the level for daily wear lens users.

People who have a history of corneal ulceration should not wear the extended wear lenses, and those who do develop ulcers should be treated with topical broad spectrum antibacterials.

Doctors have been urged to ask patients if they are contact lens users should they complain about sudden, unexplained painful eyes.