Shadow Europe minister Graham Brady has resigned from the Conservative front bench, amid the row over the party's attitude towards grammar schools.
Mr Brady, a staunch defender of grammar schools, said "in conscience" he had to resign to be free to speak his mind.
He has opposed the idea put forward by the party leadership that the schools do not boost social mobility.
He said he regretted any embarrassment to the Tories, but had to do what he thought was right for his constituents.
"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 option open to me," he said.
He said it was a difficult decision to quit the front bench, but said the argument that grammar schools and selection impeded social mobility was "the opposite of the truth".
"Grammar schools in selective areas are exactly the motor that does drive social mobility more effectively than comprehensive areas."
He praised the results from schools in his constituency of Altrincham & Sale West.
"And that's delivered by a wholly selective education system - it's one that educated me and it's something I've been fighting for all of my life and something I've campaigned consistently for, for 10 years in Parliament.
"Over the last couple of weeks, very sadly I've come to the conclusion that if I'm going to be free to continue to speak out in favour of selective education and grammar schools then I have to leave the front bench so that I'm not bound by collective responsibility," he said on BBC News 24.
On Tuesday he was reprimanded by the Conservative Party's chief whip and told to stick to his Europe brief.
Mr Brady dismissed suggestions that he was leaving the front bench before he was forced to go in an expected reshuffle this summer, saying he needed to be free to "speak my mind".
Party leader David Cameron, replying to Mr Brady's resignation letter, thanked him for his work as Europe spokesman, but said the party could not continue to debate whether to introduce more grammar schools.
Mr Cameron wrote: "The reason for this is to allow us to focus on the real issues in our secondary schools - namely, giving head teachers the power to ensure discipline, the need to encourage more new, good schools, the importance of setting by ability, and saving our special schools.
"These are issues which affect the children who go to the thousands of secondary schools up and down the country. They are also issues where we have clear differences with Labour."
He added that it had never been Conservative policy to undermine existing grammar schools.
Mr Brady was reprimanded after he supplied data to the Times which indicated that in areas with no selective education fewer pupils get five or more GCSEs at grade A*-C including English and maths.
That came as the row over the party's rejection of schools selecting by ability had appeared to be dying down.
Labour chairwoman Hazel Blears said his resignation showed the "chaos" engulfing the Tories.
"Their policy-making and internal discipline is a shambles," she said.
"Underneath Cameron's gloss, the Tories remain unchanged - same old right-wing Tories."
Mr Cameron has promised more city academies - the privately sponsored state schools championed by Prime Minister Tony Blair - and more streaming and setting within existing schools.
He has said existing grammar schools would not be axed, but said anyone who thought more grammar schools might be created were "delusional".
Shadow paymaster general Mark Francois is to replace Mr Brady as shadow minister for Europe, the Tories said. David Gauke will join the Treasury team.