There is a growing backlash from inside the Tory party against Cameron’s plan for a centrally chosen list of 140 “brightest and best” candidates — half of them women and many from the ethnic minorities.

However, Cameron hopes that the list will make the party more electable and regards its success as crucial to his ambition to change the party’s image in parliament from that of traditional white, middle-class men to a modern “fighting force” in touch with popular opinion. He will outline the plans in a speech this week.

Some MPs and party chairmen are already voicing concerns that it is undemocratic to force candidates chosen in London onto constituency parties who may want to make the choice for themselves.

They also fear that it will lead to a “two-tier” system in which those not on the list are seen as second-best. Some believe Cameron is deliberately picking the fight in search of a “clause 4 moment” to confront anti- reforming members — a reference to Tony Blair’s successful bid to rid the Labour party’s constitution of its old pledge to nationalise key industries.

Ann Widdecombe, the Conservative MP for Maidstone and the Weald, condemned Cameron’s idea as “diabolical”.

“It is positive discrimination by any other means. If you are drawing up a list based on merit you cannot say at the outset that 50% should be women,” she said. “Given that they are trying to get ethnic minorities and probably people of particular sexual orientation on the register also, it looks as though if you are a white, male, Christian heterosexual there are going to be very few places for you on this list.”

Gerald Howarth, Tory MP for Aldershot, said: “We need a broad range of candidates, but the rub will come when a local association makes up its mind that it wants a particular candidate who isn’t on the list.”

The party leadership is this week sending out letters to 550 Tory supporters asking them if they would like to be considered as members of the “priority list” of candidates who will be chosen for the 140 target seats the Conservatives need to win to get back into power.

The letter, seen by The Sunday Times, is signed by Bernard Jenkin, deputy party chairman, and Shireen Ritchie, chairman of the candidates’ committee and mother-in-law of the pop star Madonna.

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