Hospitals have been warned not to use thousands of breathing devices that might contain debris which could lodge in patients' airways and lungs.

Hazard notices explaining the danger caused by a manufacturing error in the devices, which are attached to ventilators, have been rushed out by government safety watchdogs.

About 14,000 breathing systems for adults and children are thought to have the fault which means small discs of red plastic from safety caps might be left loose in the equipment after the caps had been removed. These might find their way into patients' respiratory tracts.

Hospital staff and dentists have been warned not to use the potentially dangerous devices unless they have no alternatives to hand. In emergencies, they might be used, but only after full risk assessments, flushing with oxygen and close inspection. Even then, a special filter should be added to block any loose material that might remain.

The company involved, Intersurgical of Wokingham, Berkshire, estimates as few as one in 1,000 of the safety caps might contain the loose material, but is changing its manufacturing process. The medical devices agency, part of the Department of Health, insisted the number of potentially dangerous systems was small, considering similar equipment was used in 3.2 million procedures a year.

The agency said no incident involving a patient had been reported. The alarm had no connection with the apparently deliberate blocking of breathing apparatus in five hospitals last year. In one case, a nine-year-old boy, Tony Clowes, died at Broomfield hospital, Chelmsford, Essex. Police are still investigating the cases.