The potentially fatal childhood illness of whooping cough is far more widespread than official figures suggest, researchers warn.

A study of children with persistent coughs lasting more than 14 days in Oxford found almost four out of 10 had recently been infected with the bacterium, Bordetella pertussis, which causes whooping cough.

More than 85 per cent had been immunised but had still caught the infection. Researchers said the vaccine - given to infants at two, three and four months as part of the childhood vaccination programme - lessened the severity of the illness, and was important for that reason, but did not always prevent infection.

The biggest risk was to younger siblings of school-age children who were too young to be vaccinated. Half of infants under one who catch whooping cough are admitted to hospital. GPs fail to diagnose the infection in older children because it is perceived to be a disease of very young children who have the classic whoop.

Around 500 cases of the infection are notified to the Health Protection Agency in England and Wales each year but the researchers estimate that the true figure is "at least 10 times and maybe many more times higher," Richard Mayon-White, consultant epidemiologist at the University of Oxford and an author of the study, said. The study suggested the condition was "endemic among younger school-age children".

The infection, known as the "100-day cough", starts like an ordinary cold but develops into paroxysms of coughing, sometimes ending with a "whoop" and often accompanied by vomiting. The worst affected children require hospital treatment and can suffer brain damage or death.

Before vaccination was introduced in the 1960s, there were annual epidemics of between 60,000 and 160,000 cases. A scare about the safety of the vaccine in the 1970s saw vaccination rates collapse. In the 1970s and 1980s there were 200,000 cases of whooping cough and 100 deaths, a death rate of one in 2,000, according to the Health Protection Agency. Vaccination rates against whooping cough have since recovered and have remained at 94 per cent for the past decade.

In 2001, a fourth pre-school booster vaccination, given between the ages of three and five, was introduced because of concerns about the persistence of the infection in the community. The vaccine was also changed from a whole cell to an acellular version, which has fewer side-effects and can be manufactured to a higher standard.

Writing in the online edition of the British Medical Journal, the researchers urge GPs to be alert to the possibility of whooping cough in any child with a persistent cough lasting longer than two weeks.

"[Our] finding is important because secondary attack rates of pertussis in non-immunised household contacts have been estimated to be 90 per cent. Younger children are more likely than adolescents to have a new-born sibling to whom they could transmit the infection.''