The proportion of 10-year-olds who said they enjoyed the lessons has fallen from 82 per cent in 1995 to 69 per cent in 2003.
Enjoyment of maths lessons also dropped among 14-year-olds, although their interest in science remained stable. Only half said they liked maths but 85 per cent said they "valued" the subject as a route to further goals.
Derek Bell, chief executive of the Association for Science Education, said that the fall in interest in science among primary children was deeply worrying.
"We do hear that primary schools are preparing for Sats earlier, which could mean that children think science is about tests rather than anything more interesting," he said.
The view was shared by Barbara Bell, professional officer for the Association of Teachers of Mathematics, who said the emphasis on tests was partly to blame for children's boredom.
The teenagers appeared sceptical about the subjects' practical value. Just a third said that science was usually relevant to their daily lives and only a quarter expressed that view about maths.
Maths teachers were nearly twice as likely as their pupils to say their lessons had such relevance.
The Timss study also found that pupils who had more access to computers showed greater interest in both science and maths.
Bizarrely, pupils who were confident in science did better in maths, but pupils who were highly confident in maths tended to do worse in science.
A survey of the pupils tested suggested that those in England received less homework than their classmates in other countries.