Tory is first grammar schools martyr as he quits the front bench

David Cameron was last night dealt the first serious blow to his authority when a Tory frontbencher quit in protest over his controversial grammar school policy.

Graham Brady resigned as Europe spokesman after accusing Mr Cameron of undermining "excellent" selective schools in his Manchester constituency.

He is the first martyr in the row over grammar schools which has split the Conservative Party.

Up to three more front bench spokesmen are said to be considering their positions over Mr Cameron's decision to withdraw the party's traditional support for grammars.

Former grammar school pupil Mr Brady, 40, last night vowed to continue to "fight the corner" for selective education from the back benches and said that a "great many" Tory MPs supported his view.

He decided to step down as a matter of "conscience" after he was read the riot act by Tory Chief Whip Patrick McLoughlin for publishing research showing that selective schools improve the results of a whole neighbourhood.

Friends of Mr Brady said his position had become "untenable" after senior sources made clear that he would not survive the next reshuffle.

Last night, a poll found that voters now believe that the Tories are looking more disunited than Labour, with only 37 per cent saying that Mr Cameron could keep his MPs under control - compared with 40 per cent for Gordon Brown.

The CommunicateResearch poll for the Independent has also found that the Tory lead over Labour has been cut from nine percentage points to four since the Chancellor became Prime Minister-in-waiting.

In his resignation letter, Mr Brady said: "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 criticised Tory education spokesman David Willetts, who has been nicknamed "hammer of the grammars" by colleagues.

"David Willett's argument that grammar schools impede social mobility has gone much further and undermines schools in my constituency which continue to achieve excellent results for children of all abilities and from all social backgrounds," he said.

"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 told Channel 4 News: "I don't want to attack David Cameron.

"But I think what was regrettable was an argument, which was both incorrect and dangerous, was advanced by the party and that will cause signficant problems for some of the best schools in the country."

Later, he stepped up his attack on Mr Cameron's policy, branding his arguments against grammar schools "incorrect and dangerous".

Although Mr Brady is on the Right-wing of the party, he is not considered one of the "headbangers" who are being purged under Mr Cameron.

Privately, many Tory MPs are enraged by Mr Cameron's handling of the row and have accused him of "double standards" after ordering Mr Brady's reprimand.

At last year's annual conference, the Tory leader refused to sack his education spokesman Boris Johnson for controversial comments about school dinners.

He claimed at the time: "This is the Conservative Party. We are not New Labour, we don't mind if people go off message. We love it actually."

But one well-placed Tory source said: "The difference is that Boris and David go to the same sorts of dinner parties."

Tory MP Nadine Dorries's position on Mr Cameron's policy commission was also in doubt last night after she stepped up her attack on the grammars policy.

She said: "Selection provides academic children from all backgrounds with the education they need in order to equip them to compete-in the future labour market, with children who have been through the independent and public school system.

"Fact - like it or hate it, loathe it or love it, it's a fact."

In a direct attack on Mr Cameron, Mrs Dorries added: "Luck is a nonmany-transferable commodity. In the independent sector they don't leave anything to chance, neither should we.

"Basing our future education policy on untried and untested city academies is doing just that."

Tory MP Mark Field, one of Mr Brady's allies, welcomed his "brave" decision to quit.

He said: "His position had become untenable. He feels very strongly about this issue - as do of us - and he has done the right thing. Behind the scenes, there is a lot of disquiet."

Mark Francois was last night appointed to replace Mr Brady as Europe spokesman.

In a letter to Mr Brady, Mr Cameron said: "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."