The Conservative Party’s difficulties in finding a suitable candidate for the London mayoralty were exposed last night as Sir John Major declined David Cameron’s invitation to stand.

Mr Cameron gave a surprisingly enthusiastic endorsement of the idea of the former Tory Prime Minister throwing his hat into the ring. But it emerged that the two had not discussed the proposal and last night Sir John’s office said that he believed his “political future is behind him”.

Sir John’s rejection of Mr Cameron’s offer follows hard on the heels of the Conservative leader’s failure to persuade Greg Dyke, former Director-General of the BBC, to run.

Mr Cameron’s readiness to endorse Mr Dyke earned him strong criticism from the Right of his party, and his willingness to get Sir John to stand adds to the impression that he is unhappy with the choice of 40 would-be Tory mayoral candidates. Nick Boles, the former director of the think-tank Policy Exchange, appears to be the front-runner but MPs said that the recent machinations were hardly good news for him. “If you were Nick you would not detect much enthusiasm at the top,” an official said.

Matthew Parris, the Times columnist, wrote this week that Sir John should represent the Conservatives, although this was done without discussion with Sir John. Questioned about the prospect, Mr Cameron said: “John Major would be an excellent candidate. I would be delighted if he put his name forward. He’s a Londoner of great distinction. I still see a bit of him these days – he organises some things for me, which is very helpful.”

Asked whether he would try to persuade him to stand, Mr Cameron said: “Yes, I hadn’t previously thought about him but I think he would be a very good candidate.” Sir John’s spokesman said that the former Prime Minister had no intention of returning to politics. “That’s extremely generous of David Cameron, but no discussion has taken place between them,” he said. “Although Sir John has, and always will have, huge affection for London, he believes that his political future is behind him. This has never been discussed by the two of them, nor is it likely to be.”

Mr Cameron also paved the way for the frontbench return of Patrick Mercer, who was sacked after he suggested in an interview with Times Online that racial abuse was a normal part of life in the Army.

Mr Cameron said: “I think Patrick is a very talented MP. I’m sure there is room for him in a future Tory government.”

Mr Mercer, who was the Tory spokesman on homeland security, remarked that he had known many “idle and useless” ethnic minority soldiers while serving in the Army. He also said that it was normal for an ethnic minority soldier to be called a “black bastard”, only to claim later that he had been misinterpreted. He was fired by Mr Cameron shortly after the interview was published.

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