More than 350,000 Year 6 pupils have already begun revising for next summer's Sats, according to a study from Manchester University.
A survey of 465 teachers and headteachers found that 60 per cent of schools now begin test preparation in the second half of the autumn term.
Professor Bill Boyle, of the Centre for Formative Assessment Studies, found that 38 per cent of schools were already spending up to an hour a week on practice papers or revision lessons by the second half of the autumn term. A further 14 per cent spent two hours a week on test preparation, and 9 per cent spent three hours or more.
By the second half of the spring term, two-thirds of primaries spent three or more hours a week drilling pupils for the English, maths and science tests.
Professor Boyle said: "Why are we still doing this? Why do we have this obsession with tests? These figures are far too high. But teachers will keep on while the system remains in place."
The survey was carried out in the 2006-07 school year. Three quarters of schools said that the time they devoted to test preparation had increased over the past 10 years.
A study by The TES in 2002 found that only one in seven schools started test preparation in the autumn term.
Professor Boyle's survey also found that time spent on homework had increased. The proportion of schools asking pupils to spend two or more hours a week revising for tests at home rose from 9 per cent at the beginning of the school year to 30 per cent by the Easter holidays.
Almost nine out of 10 teachers said the curriculum had been narrowed by the focus on tests, 69 per cent thought moderated teacher assessment would be a reliable alternative, and 32 per cent would like to see the key stage 1 model of teacher assessment informed by KS2 test results.
David Tuck, head of Dallow Primary in Luton and past president of the National Association of Head Teachers, said: "As far as we're concerned, we start booster classes after Christmas. We have children coming in sometimes on Saturday morning or lunchtimes. The focus on tests does create a very narrow curriculum and we have to ask if this is the best thing for children.
"One researcher was asking children about their levels and a boy said to him, `You don't want to know my level, I'm a nothing.' What have we done to children? Where are we going? We need to instil confidence."