Exam results boosted by the grammar effect

Extra grammar places for pupils in areas which already operate the 11-plus examination could improve academic achievement in those authorities.

The findings, from researchers at the respected Centre for Economic Performance, cast doubt on Conservative policy on grammar schools after the party abandoned support for selective education.

David Cameron has said he will not expand grammar places unless population growth demands it.

Labour has barred grammar schools from expanding despite insisting popular and successful schools should be allowed to take in more pupils.

The centre's study took place in Northern Ireland, which retained the 11-plus as grammars schools were dismantled across the rest of the UK during the 1960s and 1970s.

It found sending more pupils to selective schools dramatically raised achievement.

Major reforms to school admissions in the province in the late 1980s boosted the proportion going to grammar schools from 31 per cent to 35 per cent.

This, according to researchers, resulted in an increase of 17 percentage points in the number of pupils achieving five or more GCSEs at grades A* to C across Northern Ireland overall.

There was also a 12 point increase in the numbers achieving at least one A-level.

They believe there is "something positive" about grammar schools which raises attainment - possibly their ability to attract good teachers or teach bright pupils together.

But the findings will do nothing to quell Conservative infighting over policy on selective education since the researchers also found evidence that grammars can have negative effects.

The findings also suggested selection at 11 disadvantages poor families, as Tory education spokesman David Willetts has claimed, with a potential cost to the economy.

Researcher Dr Sandra McNally, a CEP fellow based at the London School of Economics, said the study suggested grammar schools should become "less elitist" to raise attainment across the board.

Grammars in the ten remaining fully-selective authorities in England typically take just 25 to 30 per cent of their local populations.

Expanding the number of places in these areas could raise overall achievement, just as it did in Northern Ireland, Dr McNally said.

However further research would be needed to rule out the possibility of negative consequence for neighbouring local authorities.

"If you have a selective system in an area as things stand, allowing more grammar schools or expanding existing grammar schools could raise average performance," she said.

"This is essentially making grammar schools less elitist."

Dr McNally, who conducted the study with Professor Eric Maurin, of the Paris School of Economics, added: "When you expand the academic track, people do better, which suggests there is something good about grammar schools, something positive about the schooling experience there.

"But when the academic track is too elitist, there is a big cost.

"Disadvantaged people are much less likely to get in, and if poor people are prevented from going in the first place because they are less likely to pass the 11-plus, it suggests grammar schools entrench social disadvantage.

"People lose out from not having that opportunity, and overall that has a bad effect on the economy.

"Making them a bit less selective seems to have a positive effect, but we don't know how far down that road you can go."

David Cameron has suffered the biggest challenge to his authority as Tory leader since ditching traditional Conservative commitment to grammar schools.