The row over David Cameron's policy on grammar schools has prompted the resignation of Europe spokesman Graham Brady.
The MP resigned from his frontbench post following reports he was to be sacked for his opposition to his party's new education policy.
Cameron moved swiftly to replace the departing minister, announcing that Mark Francois will take over the role of shadow Europe minister.
David Gauke will join the Treasury team, filling Francois' previous position.
Regret
Brady had been "severely reprimanded" by party whips after he attacked shadow education secretary David Willetts for dropping Tory support for new grammar schools.
Willetts' decision that the Conservatives should back Labour's city academy programme instead provoked anger among Conservatives, including Brady who wrote a newspaper article criticising the move.
There were also reports that Cameron was set to sack Brady when he reshuffles his frontbench team to take on Gordon Brown.
Brady angered Cameron after collating data which showed that GCSE results are significantly better in areas that have an element of selection - with ethnic minority children benefiting most.
In a statement Brady said: "Faced with a choice between a frontbench 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."
In a letter to Cameron, the MP for Altrincham and Sale West expressed his "sincere regret" at having to leave the frontbench.
He insisted he was committed to the party's policies on Europe, and particularly its opposition to the EU constitution.
"In recent weeks, however, it has been increasingly difficult for me to reconcile my duties as a constituency MP with the collective responsibility that is required of members of the frontbench," he added.
"Although you made it clear when you became leader that you were not planning to introduce more grammar schools, unfortunately David Willetts' argument that grammar schools impede social mobility has gone much further and undermines the schools in my constituency which continue to achieve excellent results for children of all abilities and from all social backgrounds."
Focus
The resignation is the first major blow to Cameron's attempts to overhaul the party's policy agenda.
The Conservative leader has, however, made clear that the policy on grammar schools will not be reversed.
In a letter to Brady, the Conservative leader wrote: "I respect your decision, and would like to take this opportunity to thank you for all you have done in that role.
"Two weeks ago you accepted that we should not continue to debate whether to introduce more grammar schools.
"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.
"As you know, it has never been our policy to undermine existing grammar schools. As David Willetts and I have both said, they are good schools, and we support them.
"As you said in your statement of a few weeks ago, the priority of the next Conservative government will rightly be to ensure that children in all our secondary schools reach their true potential."
