Babies born prematurely have a much higher risk of developing severe mental disorders including psychosis, bipolar disorder and depression, according to a study to be published on Monday.

Scientists in Britain and Sweden found that people born very prematurely - at less than 32 weeks' gestation - were three times more likely than those born at term to be hospitalized with a psychiatric illness at aged 16 and older.

The researchers think the increased risk may be down to small but important differences in brain development in babies born before the a full 40 week gestation period.

The risk varied depending on the condition - psychosis was 2.5 times more likely for premature babies, severe depression 3 times more likely, and bipolar disorder 7.4 times more likely for those born before 32 weeks.

The study, to be published in the Archives of General Psychiatry journal, also found smaller but significant increased psychiatric risks for babies born only moderately early, at between 32 and 36 weeks.