Image caption More than 13,000 primary schools will have to convert to academies, under government proposals

An analysis of primary school test results in England suggests that successful schools are not likely to improve when they become academies.

But the study, by education data firm SchoolDash, says there can be gains for disadvantaged pupils in struggling primary schools that convert.

Most secondary schools are academies, but only about one in six primary schools has changed status.

The study comes as the government wants all state schools to become academies.

Research into the achievement of academies has tended to focus on secondary schools, whose numbers have risen over more than a decade.

But if all schools are forced to become academies, the greatest impact will be among primary schools, where more than 13,000 schools will need to change.

This SchoolDash analysis compared the performance in test results of academies with local authority primaries in similar circumstances, such as levels of deprivation.

Image copyright PA Image caption Nicky Morgan faced challenges from head teachers over forced academies

Most primary academies are "converter" academies, which tended to be high-achieving before becoming academies.

The research found "no evidence" that academy status brought improvements to these schools or that such schools were any better than their local authority counterparts.

But the smaller number of "sponsored" academies, more likely to have been previously under-achieving, made more progress when they became academies.

These improvements were "especially noticeable for disadvantaged pupils", said SchoolDash founder Timo Hannay.

"The challenge for those who support compulsory academisation is to explain why a good school should be forced to convert against its wishes if there are unlikely to be any tangible academic gains," said Dr Hannay.

"And the challenge for those who oppose it is to explain how else they would reduce the disparity between the best and the worst schools in the country, a gap that is still far too wide."

The study also shows the wide regional variations in the spread of academies.

There are some authorities where more than half of primary schools are already academies, including Bournemouth, Darlington and Middlesbrough.

And there are other authorities, such as West Berkshire, Camden and Brighton and Hove, where there are no converter academies.

The analysis shows that in the past couple of years, the numbers of primary schools choosing to become academies has been falling.

But the proposals from the government will mean that all primary schools will have to become academies.

This has proved controversial with some of the government's own backbenchers, who have been unconvinced about why outstanding schools should be compelled to become academies.

Education Secretary Nicky Morgan speaking to the education select committee last week, said that "fear of the unknown" had deterred primary schools from becoming academies.

But she said moving to a single type of state school would create a "strong, consistent system".

Head teachers have threatened industrial action over the plans and heckled Education Secretary Nicky Morgan at the National Association of Head Teachers' conference at the weekend.

Russell Hobby, the NAHT's general secretary, said: "There are times when it is right to make controversial and costly changes. Those are times when you have compelling evidence that they will make things a lot better. We do not have such evidence.

"There are good academies and struggling academies; just like maintained schools. There are outstanding chains and weak chains; just like local authorities.

"The use of evidence in the academy debate is flawed because we rarely compare like-for-like.

"You should not compare converter academies with the national average because they were above average before they converted.

"You would expect higher attainment but slower improvement. You should not compare sponsored academies with the national average because they were below average before they converted. You might expect lower attainment and faster improvement."