Three-quarters of local Conservative associations are losing activists as the party suffers a recruitment crisis which has seen membership halve since David Cameron became leader.
The latest estimates put Conservative membership at between 130,000 and 170,000, compared with almost 300,000 shortly after Mr Cameron succeeded Michael Howard. There are fears within the party that on current trends it could soon fall below 100,000.
All major parties are having an uphill struggle to attract foot soldiers to run local associations and to campaign in local and general elections. The collective membership of the Tories, Labour and the Liberal Democrats has fallen by three-quarters since the late 1980s, while involvement in organisations such as the National Trust has soared.
The recruitment problems are most acute for the Conservatives, who claimed a membership of nearly three million in the 1950s and over one million until 1990. In a survey of Tory Party members, 76 per cent said membership was falling in their local parties, 14 per cent thought it was unchanged and 10 per cent said it was increasing.
There is anecdotal evidence that many desertions have been promoted by Mr Cameron's strong support for same-sex marriage. Others are angry about policy compromises forced on the Prime Minister from governing in coalition.
Tim Montgomerie, editor of the ConservativeHome website, which conducted the survey, said: "Cameron's compromises on traditional Tory beliefs and the failure of those compromises to deliver a parliamentary majority mean he's upsetting both kinds of grassroots member." A Tory spokeswoman insisted participation in the party was "as strong as ever".
Labour's membership now stands at 187,000, a rise of 31,000 since Ed Miliband succeeded Gordon Brown, but still half the level of Tony Blair's early days as leader. Liberal Democrat membership is around 60,000, a fall of about 5,000 since the last general election, but about the same level as three years ago. The UK Independence Party has seen its membership rise from 16,500 in 2009 to almost 19,000.