Conservative Party chairs have warned that David Cameron has damaged his own party with the same-sex marriage bill, which is due to be debated in the House of Commons tomorrow.

In a letter to the Tory leader, 30 past and present local Conservative chairs from the party’s Grassroots organisation warned that his push for same-sex marriage had made winning the general election “virtually impossible”.

It stated that marriage equality had led to Tory voters switching to UKIP, and warned they would not switch back until the Conservatives abandoned the policy “or the party leadership changed”

Conservative Grassroots chair Bob Woolard said: “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.

“The marriage-based family is at the heart of Conservatism,” he added. “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 should also strengthen conventional marriage.”

According to the letter, Cameron’s “refusal to listen to reason and grassroots opinion is causing many previously loyal Conservatives to leave the party – some are lost forever and many will not contemplate re-joining unless the Bill is abandoned or the party leadership changed”.

It added that minority group Conservative supporters were among those most alienated by the bill, as they “cannot comprehend how a Conservative prime minister can be promoting a Bill that will redefine marriage in a way which is contrary to their religious and cultural beliefs and practices”.

Yesterday openly gay Conservative MP Mike Freer spoke out to say that he thinks other Tory MPs are trying to “derail” the equal marriage bill, and that an amendment to allow opposite-sex civil partnerships could undermine it.

Last week Defence Secretary Philip Hammond told BBC1′s Question Time, regarding same-sex marriage: “There was no huge demand for this and we didn’t need to spend a lot of parliamentary time and upset vast numbers of people in order to do this.

He added: “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.”

A spokesman for David Cameron rejected the claim that too much time had been spent on the issue.

He said: “The prime minister thinks that the right process is being followed. The government has set out the legislative programme and it is the right one.