Theresa May would not have the support of enough Conservative MPs for Parliament to give the green light to a new wave of grammar schools, a party backbencher has said.
Mark Pritchard (pictured) said the rumoured plan would lack political legitimacy because it was not in the Tories' 2015 general election manifesto, and he suggested that the prime minister had no mandate to bring it forward.
Opposition parties have reacted with fury to reports that selective education may be back on the agenda, vowing to fight a system which Labour said should be consigned to "the dustbin of history".
The Liberal Democrats have also vowed to fight the proposal, leaving the Tories relying largely on their working parliamentary majority of 17 MPs.
Mr Pritchard's comments followed reports that the prime minister could announce that she is lifting the ban on new grammar schools as early as the Conservative conference in October .
Downing Street has not denied the claims, but said only that any change in policy would be announced "in due course".
But Mr Pritchard told BBC Radio 4's World At One: "I don't think there's any political legitimacy for the policy and I doubt there would be enough support within the parliamentary party in the Commons.
"It would be a significant shift in Conservative education policy and, personally, I'd have to see the detail of any Bill, and I suspect it would have to be a Bill in a new Queen's Speech."
The Wrekin MP added: "It wasn't in the party manifesto, it therefore lacks political legitimacy, and I doubt it would have the support of the parliamentary party.
"That said, [existing] grammar schools should be allowed to expand."
Greening 'open-minded' on selection
Education secretary Justine Greening last month confirmed that the issue was in her "in tray" for consideration, and said that she was "prepared to be open-minded" about school selection.
But she signalled that this might not mean a return to the old pattern of grammars and secondary moderns by stressing that education was no longer a "binary" world and that there was already a range of different types of school on offer.
Ms May is thought to be a supporter of new selective schools, having backed a grammar school's proposal to open a new "annexe" in her Maidenhead constituency.
And the prime minister's new chief of staff, Nick Timothy, has also backed new selective schools in the past.
Ms May's predecessor, David Cameron, annoyed some Conservative backbenchers by resisting the creation of new grammar schools, focusing his education policy instead on academies and free schools which do not select on ability at the age of 11.
Any return to the grammar system can be expected to be divisive.
Opponents argue that the 11-plus exam led to elite schools dominated by middle-class children while the majority of young people from poorer backgrounds received sub-standard education in secondary moderns.
Today the Sutton Trust social mobility charity called for a focus on educating "highly able" pupils in comprehensive schools.