Snogging many partners almost quadruples a teenager's risk of contracting two potentially deadly infections, researchers warned today.

According to a study in the British Medical Journal, kissing with tongues was a major factor in spreading the meningococcal bacteria, which can cause meningitis (an inflammation of the lining around the brain and spinal cord), and septicaemia (a form of blood poisoning).

French kissing with at least seven partners in a fortnight increased the risk of contracting meningitis or septicaemia fourfold, researchers said.

They found that teenagers who regularly attended religious ceremonies were at less risk of contracting meningococcal disease, probably because they were less likely to snog many partners.

The team said teenagers should change their behaviour to reduce their risk of infection, but accepted this was unlikely.

The study examined the reasons for the steep rise in cases of, and deaths from, meningitis and septicaemia among teenagers in the UK during the 1990s. The introduction of the meningitis C vaccine in the UK in 1998 helped to lower numbers, but other forms of the infection remained a major problem.

The researchers looked at the cases of 144 15- to 19-year-olds who were admitted to hospital with meningitis or septicaemia in England between January 1999 and June 2000.

The team, including researchers from the Institute of Child Health in London and the Health Protection Agency, which monitors infectious diseases, took blood samples and nose and throat swabs. They also asked patients about potential risk factors for meningitis.

They found that those who were students, who mixed with large numbers of young people, were at a greater risk of meningococcal infection.

The researchers wrote in the BMJ: "Intimate kissing has been shown to be a risk factor for the carriage of meningococci in university students and it is likely that intimate kissing with multiple partners increases the risk of transmission."

They recommended that public health campaigns warn teenagers about the snogging risk. But because this was "unlikely to have a major impact" on their behaviour, development of more vaccines remained the most effective way to combat meningococcal infections.

Linda Glennie, of the Meningitis Research Foundation, which funded the study, said: "While we recognise that lifestyle factors are the reason why young people are more at risk of contracting meningococcal meningitis and septicaemia than other age groups, we would certainly not advocate they refrain from kissing."

But she said the charity still advocated distributing health advice tailored for older teenagers to colleges and universities.