David Cameron is facing a grassroots campaign that looks set to scupper his plans of boosting the number of Tory women MPs by imposing all-women shortlists.

The Tory leader provoked a furious backlash from his party's membership and backbenchers last week after announcing that he had reversed his opposition to the policy, used by Labour to increase its number of female MPs since 1997.

He said that he would impose the all-women lists in some constituencies in which the sitting MP announced, after 1 January, that they were stepping down.

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Mr Cameron has been worried that high-quality women candidates have been failing to win safe seats, despite current rules demanding that at least 50 per cent of all shortlists must be made up of women.

Several female candidates favoured by Conservative Central Office, such as healthcare expert Julia Manning, who failed to win the right to represent Bracknell recently, are still waiting for a parliamentary seat.

However, campaigners now hope to sidestep the threat of all-women lists by writing to the 161 Conservative MPs who have not yet announced whether or not they will stand at the next election, urging them to do so before the end of the year. It could block Mr Cameron from imposing any all-women lists before the next election.

Jonathan Isaby, the co-editor of the Tory members' website ConservativeHome, who sent out the letters yesterday, said he had launched the campaign to "ensure that their associations do not have to select under the restrictive by-election rules", which dictate that the CCO can impose a shortlist containing three or four names on local parties.