A deadly measles complication that kills children years after they have fallen ill is far more common than previously thought, doctors have warned.

Scientists say they are alarmed at the discovery that the neurological disorder, which is always fatal, may affect up to one in 600 non-vaccinated infants under a year old who get measles.

The condition, subacute sclerosing panencephalitis (SSPE), was once considered extremely rare, posing a risk to just one in 100,000 children with a history of measles.

Recent research from Germany updated the incidence to one in 1,700 among children infected before the age of five.