Secondary head teachers in North Somerset have written to Prime Minister Theresa May, urging her to reconsider proposals for new grammar schools.
North Somerset is one of only six areas identified by one think tank recently as likely to benefit from selection.
But, in the letter, the heads of nine local comprehensives said selection would be a “retrograde step” which would “undermine” progress.
Downing Street said ministers would respond to the letter “in due course”.
And a Department for Education spokeswoman said all children should have access to an education able to “unlock their talents”.
Last month’s report by the Education Policy Institute said there were six areas in England where parents would like new grammar schools and where creating them would benefit the wider school population.
The researchers modelled the impact of the government’s plans and applied the conditions for allowing new schools as set out in the consultation document, Schools that Work for Everyone.
These included that new grammars should not be to the detriment of pupils who do not pass entrance tests, should not undermine existing high-performing schools and should be allowed only in areas where parents want them.
The six areas that met the criteria were Solihull, Essex, North Yorkshire, Dorset, Northamptonshire and North Somerset, according to EPI.
At the time, the government called the study “a crude attempt to second-guess” the results of its consultation on new schools.
Now the heads of nine secondary schools in North Somerset say they fear the introduction of selection would “undermine the rapid progress that we have been making for the young people in our communities”.
They say they fear the effect of selective education “on thriving and popular community schools”.
They add: “No-one could object to the concept of grammar schools in isolation but they do not exist in isolation.
“Where grammars are created other schools become secondary moderns.
“”This is the very definition of a zero-sum game.
“A child can only receive his or her education in one school.
“In most cases, teachers only work in one institution.”
The heads say they fear that new secondary moderns would be less attractive places to work.
And they say this, in the light of the current teacher recruitment “crisis”, would “have a disproportionately negative effect on the most vulnerable who are being served increasingly well in the comprehensive system that exists currently.”
The heads argue that the best way to drive social mobility is to ensure that all children have the opportunity to attend a good or outstanding local comprehensive.
“The transformation of thriving local comprehensives into secondary moderns will be unpopular with parents, with teachers, and will provide a poorer experience for the great majority of young people,” the letter argues.
Gary Lewis, head teacher of Gordano School in Portishead and a signatory to the letter said North Somerset comprehensives were thriving.
He pointed out that of 11 comprehensives in the area, 10 were rated good or outstanding by Ofsted.
“Even with increasingly rigorous exams, their outcomes are increasing year on year,” he argued.
Details of the letter emerged as the boss of England’s biggest academy chain told the Times Educational Supplement that he would consider selection based on ability in his schools.
“I think that as we look at each of those schools in the particular markets and catchment areas in which they exist, there might be arguments for selection in certain places,” Julian Drinkall, chief executive of the Academies Enterprise Trust, told the TES.
“I think there will be arguments where grammar schools in certain areas make sense and there will be a number of areas where it doesn’t make sense.”
The AET multi-academy trust currently runs 66 primary, special and secondary academies.
A Department for Education spokeswoman said: “The government consultation puts forward proposals to allow more grammar school places to be created, making them a realistic choice for more parents, but only on the basis that strict conditions are met to ensure this also contributes to the improvement of other parts of the school system.”