The first "new" grammar school in 50 years has been approved, but the Education Secretary insisted that the Government has not changed its mind on selection
Weald of Kent school in Tonbridge has been granted the green light to open an "annexe" catering to about 450 pupils at a site around seven miles away in Sevenoaks.
Approval for the extension, which is expected to open in 2017, was confirmed by Secretary of State Nicky Morgan in a statement.
Labour passed laws in 1998 banning the creation of new grammars - which are selective state schools - but the Weald of Kent plan avoided the ban because existing schools are still allowed to expand if there is sufficient demand.
In her statement, Mrs Morgan insisted that she was satisfied that the proposal was a "genuine extension" of the current school, adding that approval was not a change in policy on selective schools.
But shadow education secretary Lucy Powell described the move as a "hugely backward step".
Selective state schools, such as grammars, take pupils on the basis of ability, with youngsters usually sitting exams such as the 11 plus in their final year of primary school.
There are currently 164 grammars in England, with many concentrated in counties including Buckinghamshire and Kent.
Mrs Morgan's statement said: "I am satisfied that this proposal represents a genuine expansion of the existing school and that there will be integration between the two sites in terms of leadership, management, governance, admissions and curriculum.
"I am also satisfied that the excellent quality of learning currently delivered will be replicated across the newly-expanded school."
She added: "My decision in this case has been taken on the basis of the proposal from the Weald of Kent, in line with legislation and criteria determining what constitutes an expansion.
"It does not reflect a change in this Government's position on selective schools.
"Rather it reaffirms our view that all good schools should be able to expand, a policy which is vital to meet the significant increase in demand for pupil places in coming years."
She said that further extension applications from good selective schools will be considered under the current laws and would have to show that they are a genuine expansion.
A previous proposal to expand Weald of Kent Grammar was turned down because it was for a mixed sex school when the existing school was girls only.
Under the revised plans, girls will be taught on both sites with a mixed-sex sixth form.
Kent County Council leader Paul Carter said he was "very pleased" that the expansion has been given approval.
"Weald of Kent Grammar School had been constrained by not being able to expand on its existing site because, frankly, it's full up. Therefore, expansion on an alternative site in Sevenoaks, I believe to be the right thing to do."
Many of the school's pupils are from Sevenoaks and the new annexe will be beneficial for students and parents, he added.
Selecting pupils on the basis of ability has long been controversial, with strong opinions on both sides of the debate.
Campaigners in favour of more grammar schools argue that they provide a high-quality education for bright pupils regardless of their background, while critics say that entry to selective schools is often restricted to those wealthier youngsters whose parents can afford private tuition or extra help, with those from poorer backgrounds more likely to miss out.
In her statement today, the Education Secretary said: "It is this Government's policy that all good and outstanding schools should be able to expand to offer excellent places to local students.
"The Weald of Kent Grammar School is one of the top performing schools in the country, with 99% of its students achieving five A*-C grades in GCSE exams in 2014, and 98% of sixth form students achieving at least three A-Levels at grades A*-E."
Ms Powell said: "Having made social mobility the centre of his conference speech, David Cameron should look at the clear evidence on grammar schools: They do not increase equality of opportunity, they make it worse.
"Tiny numbers of children from disadvantaged backgrounds pass their tests because they are the preserve of the privately tutored."
Research published by the Sutton Trust charity two years ago concluded that less than 3% of grammar school entrants are entitled to free school meals - a key measure of poverty - while almost 13% came from outside state education. These were largely believed to be youngsters who had attended fee-paying prep schools.