The Government's own research has cast fresh doubt on Ministers' claims of rising standards in primary schools.

The widespread practice of drilling 11-year-olds to pass their national curriculum tests was "likely" to have boosted results, an investigation for the Education Department found.

But the study suggested that so-called "teaching to the test" in this way may not help children's longer-term learning. It called for further research on the issue.

The report comes just days before the Government publishes this year's league tables for primary schools in England, based on the national tests in English, maths and science, often known as 'Sats'.

It follows a long-running row over how far primary school results reflect real improvements in children's abilities in reading, writing and maths. Critics argued that results improved because teachers were teaching pupils what they needed to know just to pass the tests.

The report said: "Intensive preparation for and practise of national curriculum tests appeared to be seen in all schools as a natural thing to do. Without question national tests dominated classroom teaching of both subjects (English and maths) in these schools for a large part of year six."

The study said targets were a factor in the tendency "to focus teaching on the requirements of the national curriculum tests". "This appears to be something that both teachers and pupils understand and regard as a fact of life.

"It is likely to have contributed to the increase in pupils achieving Level 4 (the standard expected of 11-year-olds) since the introduction of national testing.

"Whether it leads to sustainable learning is another matter, which should be a priority for further research."

The report, Teaching Approaches to Promote Consistent Level 4 Performance in Key Stage 2 English and Mathematics, was conducted by academics at Durham University.

It was intended as a way of identifying which teaching methods helped 11-year-olds do well in the national tests.

Researchers conducted a series of in-depth case studies at nine schools selected for their high test results.

Headteachers regard these tests as crucial because they form the basis for the league tables which parents use to compare neighbouring schools.

Steve Sinnott, general secretary of the National Union of Teachers, welcomed the report's call for more research into whether the tests damaged children's long-term education. He said: "It has always been a concern of the NUT's that the high stakes nature of these tests and their conversion into league tables would result in teaching to the test."