Creating more grammar schools is likely to widen the attainment gap between rich and poor children, and there is no evidence that it will raise overall education standards, a report has warned.
An expansion of grammar schools in areas which already have a large representation of selective schools is likely to lead to "small but growing attainment losses" for those not attending selective schools, research by the Education Policy Institute (EPI) shows.
This would be greatest among poor children, who are already under-represented at grammar schools, the independent research body said.
The gap between children on free school meals (FSM) achieving five A*-C grades at GCSE, including English and maths, and other children is six percent wider in selective areas than in non-selective areas, the study found.
At a national level there did not appear to be a significant attainment penalty from not attending a grammar school, the EPI found.
The EPI also warned that it would be "very challenging" to significantly improve grammar school access for poorer children as 60% of the attainment gap takes place before the age of 11, when children sit the entrance test.
The report also suggested that the resources which might be used to create additional grammar schools could be deployed to help create high-attaining non-selective schools, which are much more socially representative than grammars, it suggested.
The first generation of academies sponsored by Labour are now helping to educate around 50,000 FSM children, the organisation said, compared with 4,000 in grammar schools.
Mary Bousted, general secretary of the Association of Teachers and Lecturers (ATL), said: "This report could not be more timely, and its conclusions could not be more devastating for the Government - that there is no evidence that overall educational standards in England would be improved by creating additional grammar schools.
"The Government should listen to the evidence and ditch its intention to use 1950s' solutions to 21st century problems."
Malcolm Trobe, interim general secretary of the Association of School and College Leaders, added: "It is vital that Government policy is based on evidence rather than nostalgia and anecdote."
It added that the "misguided" policy was a "dangerous distraction" from the critical issues of a teacher-recruitment crisis and funding pressures.
A Department for Education spokesman said that it would ensure new selective schools prioritise the admission of pupils from lower income households or support other local pupils in non-selective schools to help raise standards.
"We know grammar schools provide a good education for disadvantaged pupils, helping to all but eliminate the attainment gap between disadvantaged pupils and their peers, and we want more pupils from lower income backgrounds to benefit from that."