Conservative European spokesman Graham Brady has left the front bench following a clash with party officials over grammar schools.
Earlier this month shadow education secretary David Willetts affirmed the Conservatives' support for city academies and refusal to build any more grammar schools.
This created a rift in the party, with Tory Europe spokesman Graham Brady quoted in the Times pointing to data which appears to support academic selection. "These facts appear to confirm my own experiences: that selection raises the standards for everyone in both grammar and high schools in selective areas," he told the paper.
"It is vitally important that policy should be developed with a full understanding of all of these facts, which might lead to the introduction of selection in other ways."
The public dissent led to a "severe reprimand" for the Conservative front bencher. Mr Brady has reportedly been told to "stick to his brief" as Europe spokesman.
This was enough for Mr Brady to leave his role as European spokesman, with his parliamentary office confirming to politics.co.uk that he has resigned over the matter.
"Faced with a choice between a front bench position that I have loved and doing what I believe to be right for my constituents and for the many hundreds of thousands of families who are ill-served by state education in this country; there is in conscience only one decision open to me," he said.
"This has been a very difficult decision, I will continue to serve the interests of the Conservative cause in Greater Manchester and nationally to the best of my ability and I wish David Cameron well in returning the Conservative party to government."
Pushing for academic selection to drive up educational standards has been a key pillar of Tory policy for decades, but David Cameron's new-look party has shifted support for grammar schools.
Mr Brady, in his resignation letter to Mr Cameron, noted that his Altrincham and Sale West constituency there were four grammar schools and four high schools.
He added that Mr Willetts' argument that grammar schools impeded social mobility "undermines" the schools in his constituency and he "must now help to provide the evidence that those schools need to defend themselves".
"If I did not put this evidence in the public domain, I would be failing the excellent schools in my constituency. Furthermore at a time when both major parties have adopted very similar education policies, the quality of public debate would be diminished if no one in the House of Commons were able to bring forward evidence that may lead to better educational outcomes for the nation's children," Mr Brady argued.
Mr Brady is far from alone in his party in questioning the move away from academic selection, with the former grammar school pupil calling for an increase in grammar schools in inner cities.
He released figures to the Times showing in areas with no selective education 42.6 per cent of pupils failed to achieve five or more GCSEs at grade A*-C including English and maths.
However, in partially selective areas this rises to 46 per cent and on to 49.8 per cent in areas where all pupils take the 11-plus.
The Conservatives question the figures, pointing out that areas with selection such as Kent cannot be directly compared with inner cities where there is no selection.
"We support existing grammar schools and that has never been in doubt," Mr Willets explained when the row erupted.
"But, returning to 11 plus will not increase social mobility, and we should focus on raising standards in 3,000 secondary schools."