Bridget Phillipson confirmed Sir Keir Starmer’s party would seek to block the creation of new selective schools and stop existing ones from taking on greater numbers of students.
Ms Phillipson insisted Labour would not seek to abolish any grammars but focus instead on improving standards in state education in England.
“I wouldn’t begin with a system that involves selection at 11, but we are where we are,” she told Times Radio.
“My priority as secretary of state wouldn't be to see big structural upheaval in our schools, given the pressure that they're under, but making sure that we do deliver a brilliant state education for every child in our country.
“We do not support expansion of grammar schools. I'm saying that we wouldn't seek to significantly change the existing system that we have. But we absolutely oppose any expansion.”
Asked why she opposed the creation of new institutions, Ms Phillipson replied: “I don’t think that should be the priority. It should be about driving up standards in the schools that we already have.”
She revealed that a Labour government “won’t allow expansion to take place”, even where it is currently planned to do so.
Maidstone Grammar School for Girls will soon take on an extra 30 pupils per year and replace two of its blocks of classrooms after Kent County Council approved an expansion worth £7.8 million in August.
Ms Phillipson also branded Rishi Sunak’s defence of tax breaks for private schools “indefensible” in the wake of his row with Sir Keir, the Labour leader, about aspiration at Wednesday’s Prime Minister’s Questions.
However, she added: “I think parents will choose to do what's right by their children. I'm not suggesting we deny parents that opportunity.”
Labour railed against a planned selective education push by Theresa May under Jeremy Corbyn, Sir Keir’s predecessor, who claimed her plans were “divisive”.
“Grammar schools depress overall educational achievement and siphon off a few better-off children at the expense of the rest,” he said at the time.
There was anger in Conservative ranks last week after Mr Sunak abandoned plans to expand grammar schools that had been drawn up by Liz Truss, his predecessor.
Baroness Barran, a Department for Education minister, said the priority was ensuring an “outstanding education” for as many children as possible – regardless of their ability – “rather than creating more grammar schools”.
This came despite polling by the think tank Onward showing that two-thirds of Tory voters supporting grammars and almost half agreeing more should be built.
Backbenchers including Jonathan Gullis and Sir Graham Brady, the chairman of the 1922 Committee, now hope to pressure Mr Sunak into an about-turn.