Blocking the expansion of a successful grammar school would have been "perverse", the education secretary has claimed.
Nicky Morgan defended last week's decision to allow the Weald of Kent grammar in Tonbridge, Kent, to open a new site as part of a major expansion.
She told MPs the plans were clearly an expansion of the same school, with pupils and teachers due to work across both sites.
But shadow education secretary Lucy Powell condemned the plans as allowing the first new taxpayer-funded grammar school in 50 years, warning that the decision would "open the floodgates".
Making a parliamentary statement, Ms Morgan said: "It would have been perverse, given the need for more good and outstanding school places, to have rejected this application for expansion purely on the basis that the school in question is a grammar school.
"This was a decision to approve a proposed expansion of this school that was taken on the facts in this case and it is my firm belief we should not stand in the way of good schools, all good schools, being able to expand."
She added: "I realise there has been significant interest in the outcome of this case, including from MPs, but I would like to take this opportunity to confirm that the government has no plans to change our policy on grammar schools.”
Ms Powell rejected the education secretary's assertion that the existing school was expanding.
She told MPs: "The truth about selective grammar schools is that far from being the bastions of social mobility that some romanticise about, they have entrenched social advantage.
"Today's grammar schools cannot deny that selection criteria favour the privately tutored and those with the means to acquire that tuition.
"This decision to allow a so-called annex – 10 miles away from an existing school in a different town – is what everyone knows it to be: a new school. It will the first new grammar school to open in more than 50 years."