David Cameron's "bizarre drive" to legalise gay marriage has made it "virtually impossible" for the Conservatives to win the next general election, his party activists have warned.
Bob Woollard, chairman of the Conservative Grassroots umbrella group of local associations, led a group of 30 local party chairmen to Downing Street on Saturday 18 May to convey their views to the prime minister.
They handed him a letter on behalf of the organisation warning that his backing for gay marriage was driving traditional Tory voters to switch to Ukip.
MPs will debate the Marriage (Same Sex Couples) Bill for two days from Monday, with many Tory MPs expected to oppose the legislation.
The letter said: "The marriage-based family is at the heart of Conservatism. This dilution and unravelling of marriage has de-motivated many ordinary, loyal Conservative Party members and has undermined their years of hard work for something they believed in.
"It makes winning the next election virtually impossible. For the sake of our children they [the government] should also strengthen conventional marriage."
The letter took Cameron to task for his "refusal to listen to reason and grassroots opinion", saying his proposals were driving previously loyal Conservatives to leave the party.
It presented the prime minister with a new ultimatum, warning voters who had abandoned the Conservatives would not return unless the bill was abandoned - "or the leadership changed".
Woollard said the policy had "upset countless people and caused many, many people - hundreds, maybe thousands - to leave the party over this issue".
"The prime minister's bizarre drive to ram this legislation through Parliament, without any democratic mandate and without the support of party members has been a disaster and has driven thousands of voters to Ukip," said Woollard.
"It is vital that the PM and those around him concentrate on the issues that matter to voters - cleaning up the economic mess left by the previous government, tackling immigration and sorting out the Europe question. For the sake of our children they should also strengthen conventional marriage."
He said unless the policy was reversed it would lead to a "slippery slope downwards" with more members deserting the party, "getting utterly disenchanted and frustrated that nobody is listening to them".
The letter will pile the pressure on Cameron a day after claims emerged that one of his closest allies had characterised the party's rank-and-file membership as "swivel-eyed loons".
Sense of crisis
Compounding the sense of crisis, former chancellor Lord Howe warned the party was experiencing a "long nervous breakdown" over Europe.
Howe accused the party leadership of "running scared" of its own backbenchers, and warned that the mood of prevailing "anti-Europeanism" risked diminishing the country's influence in the world.
Writing in the Observer, Howe said: "Sadly, by making it clear in January that he opposes the current terms of UK membership of the EU, the prime minister has opened a Pandora's box politically and seems to be losing control of his party in the process.
"The ratchet-effect of Euroscepticism has now gone so far that the Conservative leadership is in effect running scared of its own backbenchers, let alone Ukip, having allowed deep anti-Europeanism to infect the very soul of the party."
However, Health Secretary Jeremy Hunt attempted to paper over the schisms that have emerged in the party over Europe and same-sex marriage.
Hunt said the prime minister was right to raise the issue even though it was "difficult" for many people. "I personally support it. I support it because I believe in the institution of marriage and I think we should be encouraging people to make a lifelong commitment to each other. I think society is stronger if you do that.
"In my own case I got married in a church and not a register office because I happened to want to make my marriage vows in front of God.
"I think if gay people want to do that, and if the church is willing to conduct that ceremony, we shouldn't stand in their way."
His remarks came after Phillip Hammond, the defence secretary, broke Cabinet ranks to acknowledge the gay marriage plans had alienated large sections of the party's support.
"This change does redefine marriage. For millions and millions of people who are married, the meaning of marriage changes," Hammond said on Thursday.
"There is a real sense of anger among many people who are married that any government thinks it has the ability to change the definition of an institution like marriage. I have never felt this is what we should be focusing on."
His comments were seen by some in the party as an attempt to position himself as chancellor or even leader in a post-Cameron administration.
Under the marriage bill, the Church of England and the Church in Wales will be banned from offering same-sex marriages because of their strongly stated opposition, unless they change canon law. Other religious organisations will be able to "opt in" to holding ceremonies.
In a separate development, hundreds of Muslim leaders attacked the marriage plans in an open letter to Cameron, Labour leader Ed Miliband and Nick Clegg, the Deputy Prime Minister.
More than 500 community leaders and imams signed the letter which stated "marriage is a sacred contract between a man and a woman that cannot be redefined".