Bernard Jenkin quit his position in a wide-ranging Conservative reshuffle as a leading ethnic minority candidate denounced his own party, claiming that Mr Jenkin had told him that he would not be selected for a seat because he was not white.

Ali Miraj, a chartered accountant, said that he was turned down by the safe Conservative constituency of Witham, Essex, after he said that Mr Jenkin told him: “Good luck Ali, but I would be shocked if they didn’t pick a white middle-class male.”

In an extraordinary outburst on his blog, Mr Miraj, a councillor who has already fought two general elections as a Conservative candidate, and who sits on two of the party’s policy commissions, said that despite great efforts to give opportunities to ethnic minorites, the party still had “a mountain to climb”. He said that sitting MPs suggested to him repeatedly that only white middle-class men would be chosen for the best seats, and that he was considering giving up the fight after reluctantly facing “up to a reality that I thought I never would”.

Mr Jenkin said that he did not recall making the comment, but Labour immediately claimed that it was just the latest proof that the Conservatives had not changed. On Monday, the party suspended one of its candidates after an e-mail ridiculing illegal immigrants was sent from her computer.

Hazel Blears, the Labour Party chairman, said: “With every passing day we see a Conservative Party that has not changed. It is incredible that candidates in the Conservative Party are being given this message by the man David Cameron has appointed to improve the representative nature of Tory candidates and the fairness of the selection procedure.”

Mr Cameron and Mr Jenkin both insisted that the move to the back benches had nothing to do with Mr Miraj’s comments and that it was just a coincidence. Mr Jenkin said that he had been offered another frontbench post, but he wanted a break after spending ten years on the Opposition front benches.

In a response to Mr Jenkin’s resignation letter, Mr Cameron wrote: “With your help and hard work we have made tremendous progress. There remains a great deal of work to do — but we have come a long way in short time.”

Mr Jenkin, who was in charge of the controversial “A- list” of priority candidates, aimed at selecting more women and ethnic minorities in safe seats, was replaced by John Maples. The party said that the list had been a success, with more than a third of candidates in target seats now being women and 12 per cent of those on the list being ethnic minorities, a higher proportion than in the population at large.

Mr Cameron used the reshuffle of junior positions to bring more modernisers and newly elected MPs into his team and to promote one of the party’s two ethnic minority MPs. Shailesh Vara has been appointed Shadow Deputy Leader of the House, making him the most senior ethnic minority member of the Opposition.

Eight MPs elected last year were promoted to the front bench, including Ed Vaizey, who became arts spokesman.

Mr Miraj — who introduced Mr Cameron at the launch of his leadership campaign — said that he was beginning to doubt the Tory leader’s previous conviction that “the values you stand for and the passion with which you fight for them is more important than the colour of your skin and/or your religion”.

He added: “It is evident that despite his [Mr Cameron’s] huge efforts to change the Party’s attitude to candidate selection, there is still a mountain to climb. What is gradually becoming clear is not that the peak may never be assailable, but that some, including myself, may out of frustration, opt to abandon the ascent itself.”

He added that Mr Jenkin’s fellow Tory MPs, John Whittingdale and Brooks Newmark, whose seats all neighbour the new constituency, had given the same advice.

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