David Cameron has been forced to rebuke his backbench MPs, as the Conservative party threatens to fracture over the issue of grammar schools and the party's commitment to academic selection.
This could be the first real test of Mr Cameron's ability to carry his traditional supporters behind his vision of a new Conservative party.
Mr Cameron acknowledged many within the Tory party are committed to the idea of segregating children by academic ability.
However, he insisted the party would "never be taken seriously by parents" if it continued advocating selection.
Support for grammar schools would become "an electoral albatross," he said in an interview with the Evening Standard.
In comments on his website, Mr Cameron criticised the "naive" elements within his own party.
The 11 plus exam - which divided all children into grammar schools or secondary moderns - was abandoned because it was so unpopular with parents, he argued, making it "meaningless" to talk about a clamour to "bring back grammars".
He continued: "It is politically naive as it just says 'we'll help a few more escape failing schools rather than turn them round for all children.'
"The way to win the fight for aspiration is to put those things that worked in grammars - aggressive setting to stretch bright pupils, whole class teaching, strong discipline to name but three - in all schools."
Mr Cameron said it was "sad" critics missed the policies that could work; such as zero tolerance of bad behaviour, an unchallengeable right to exclusion for heads and enforceable home school contracts.
"Perhaps if I put the words 'bring back' in front of some of these policies they might just get it," he added.
The shadow education spokesman David Willetts confirmed on Wednesday the Conservatives would not call for any more grammar schools.
Instead, they would support the expansion of Labour's city academies - non-selective privately sponsored schools outside the control of local authorities.
The announcement immediately provoked uproar, including what Mr Cameron branded "near hysterical" coverage in the Daily Telegraph.
Backbench MP Roger Gale described it as another move towards "socialist policies".
The scale of the opposition forced Mr Willetts to confirm the party's position today and insist Wednesday's speech had not marked a radical policy shift.
"Our position has been consistent throughout," he continued.
Mr Willetts said today: "As David Cameron made clear when he became leader, and as I reiterated in my speech on Wednesday, we support existing grammar schools and that has never been in doubt.
"But, returning to 11 plus will not increase social mobility, and we should focus on raising standards in 3,000 secondary schools."
Mr Cameron and Mr Willett's appeal to Tories followed a poll showing the majority of Conservative voters support grammar schools.
A YouGov/Daily Telegraph poll revealed 49 per cent of voters want a mix of grammar and comprehensive schools, rising to 71 per cent among Tory voters.