Grammar school students are unlikely to achieve more academically by the age of 14 than those who do not attend the selective state schools, a study has suggested.
Researchers analysed data from 883 children in England and 733 in Northern Ireland who had similar academic achievements at primary school and who came from families with similar incomes and education levels. The research team also said attending a grammar school had no positive impact on a teenager's self-esteem or their aspirations for the future.
The study by the University College London Institute of Education comes weeks after the Government announced controversial plans to pump £50m into creating more places at grammar schools.
Those who led the research said the results indicate the cash should be directed elsewhere.
Professor John Jerrim, lead author of the study, said: "Our findings suggest that the money the Government is planning to spend on grammar school expansion is unlikely to bring benefits for young people.
"Even those children who are likely to fill these new places are unlikely to be happier, more engaged at school or have higher levels of academic achievement by the end of year nine."
Co-author Sam Sims added: "Schools across the country are already hard-pressed financially. Our research suggests that the Government would be better off directing their money towards areas of existing need, rather than expanding grammar schools."
Researchers looked at the results of tests taken in English, maths, verbal and non-verbal reasoning at the ages of three, five, seven and 11, as well as a vocabulary test at the age of 14, and compared results from those who went on to attend a grammar school and those who did not.
Children were also given a series of questionnaires at the ages of 11 and 14 to gauge their thoughts on mental health, engagement at school, well-being and interaction with peers.
Josh Hillman, director of education at the Nuffield Foundation, which funded the study, said: "These findings are important because they show for the first time the impact of attending grammar schools on a wide range of outcomes, such as young people's self-confidence, academic self-esteem and aspirations for the future.
"The evidence shows that at age 14, there is no benefit to young people of attending grammar school in these respects.
"In addition, we know from previous evidence from this study that the use of private tutoring heavily skews access to grammar schools in favour of wealthier families, dispelling the myth that they increase social mobility. In light of this evidence, it is increasingly difficult to understand the Government's rationale for spending money on expanding selective education rather than on improving education for all young people."