More than 500 patients who had cataract operations up to six years ago have had further surgery because they were fitted with faulty lenses, it was revealed yesterday.

Two brands are involved, although government safety advisers believe only a small proportion of the 270,000 people a year who have cataract surgery on the NHS might need replacements because their vision has deteriorated.

Nineteen eye surgery centres have reported patients suffering impaired sight. The problem appears to have been caused by a chemical in packaging which reacted with the lenses to lead a few years later to calcium deposits.

Gloucester hospitals NHS trust is contacting 1,800 patients over fears the implants could lead to impaired vision after 20 people in the county reported problems. All patients who had cataract operations between September 2000 and April 2001 are being notified. Other hospitals have been advised by the Medicines and Healthcare Products Regulatory Agency (MHRA) to identify patients who might have been fitted with the products to establish whether more replacements are needed.

One brand involved is Aquasense, made by Ophthalmics Innovations International between December 1999 and November 2000. More than a quarter of the 868 lenses implanted in that time have had to be removed. The other is Hydroview, made by Bausch and Lomb. It supplied 88,500 lenses in packaging using the chemical between December 1997 and May 2001.

Kent Woods, MHRA chief executive, said: "MHRA is aware of reports of clouding of some types of lens, several years after surgery."

The MHRA said most patients with the brands would not have a lens that clouded over. Bausch and Lomb said calcification could occur two to three years after surgery.