David Cameron’s difficulties with his own party escalated yesterday with the resignation of his frontbench spokesman who defended grammar schools in Monday’s Times.

Graham Brady announced that he was stepping down immediately as Europe spokesman after learning from briefings to newspapers that he would probably be dismissed in next month’s reshuffle.

Mr Brady incurred the wrath of the Tory leader after suggesting that selection at 11 raised GCSE standards for everyone in the area, especially ethnic minority children. This came a day after Mr Cameron had called his grammar school critics “inverse class warriors”.

The resignation appears unlikely to end the grammar school row, with colleagues querying Mr Cameron’s handling of the episode by keeping Mr Brady in his job while allowing reports of his imminent departure to go unchallenged.

They also contrasted his dealings with Mr Brady and Boris Johnson, the higher education spokesman who has frequently embarrassed the party by stepping out of line, and suggested that this could be evidence of double standards. They believe that Mr Johnson’s gaffes are tolerated because he is closer to Mr Cameron. “After all, they go to the same dinner parties,” a well-placed Tory source said.

The party announced that Mark Francois, the Treasury spokesman, would replace Mr Brady, and David Gauke, who entered Parliament in 2005, would join the Treasury team.

Mr Cameron faced a further challenge yesterday from a Tory MP on the public services commission, which is deciding the future of the party’s education policy. Nadine Dorries, the Mid Bedfordshire MP, wrote on her website that Mr Brady’s research “proved” that grammars benefited everyone.

“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 letter to Mr Cameron, Mr Brady said that he had decided to go because the party was undermining the four grammar schools in his constituency by claiming that they impede social mobility.

“Like all Members of Parliament who have grammar schools in their constituencies, I must now help to provide the evidence that those schools need to defend themselves. That is why I have felt duty-bound to continue my practice of obtaining and publishing facts which demonstrate that selective local education authorities perform better than comprehensive ones.”

He added: “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 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 later told The Times that he had resigned now because he would be speaking out again, and “I think it is more honourable to go now”.

On Sunday Mr Cameron declared that the row was over, but it was revived the next day by Mr Brady. He released data to The Times which indicated that in areas with no selective education 42.6 per cent of pupils get five or more GCSEs at grade A*-C, including English and maths. This compared with 46 per cent in partially selective areas and 49.8 per cent in areas where all pupils take the 11-plus.

Some frontbenchers are concerned that the party is failing to rebut Mr Brady’s argument effectively. A Tory spokesman said that the data he presented was misleading because it failed to take account of the fact that grammar schools are in wealthier parts of the country.

In his reply to Mr Brady, Mr Cameron said that he had broken his promise to stop talking about grammar schools two weeks ago.

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