David Cameron made a “catalogue of mistakes” in a letter claiming the public supports redefining marriage, according to the Coalition for Marriage (C4M).
The group warns the letter, to a former Government minister, is “misleading” and “riddled with errors”.
Mr Cameron falsely claimed all the polls show support for redefining marriage. In fact, the polls show the Conservatives could lose up to 1.1 million votes over the issue.
Mr Cameron’s letter also refers to a survey showing 7 per cent of Conservative voters are more likely to back the party and 10 per cent are less likely.
“Unfortunately for the PM”, C4M says “these figures refer to disaffected Lib Dem voters only”.
It adds: “The poll actually concludes that the policy is deeply unpopular with those who voted Conservative in 2010.”
Campaign Director Colin Hart said: “The PM is deluded if he thinks that the polling shows this undemocratic policy is popular with voters. Quite the opposite is true.
“The polls show that a majority of voters do not support the redefinition of marriage, including those in Mr Cameron’s own constituency of Witney.
“Even amongst the gay community the policy is not popular with less than one in five believing the PM is proposing this change for the right reasons.”
Mr Hart also commented: “Only last month ComRes found that 51 per cent of the public agree that marriage should continue to be defined as a life-long exclusive commitment between a man and a woman, with only 36 per cent disagreeing.
“But other polls show that if people are told that all the legal rights of marriage are already available through civil partnerships, the number supporting retaining the traditional definition of marriage rises to 70 per cent.”
He added: “The Prime Minister’s letter talks of recent polls, but they actually date back to March. And one of the key polls that he cites is quoted in a misleading way by deliberately excluding a large group of people.
“The YouGov Sunday Times poll did indeed find that 43 per cent would support same sex marriage, but the Prime Minister didn’t mention that even more were against it.
“Some 32 per cent were against gay marriage but in favour of civil partnerships and there were an additional 15 per cent who opposed both gay marriage and civil partnership with the result that same sex marriage is opposed by 47 to 43 per cent.”
He concluded: “The polls show that the public do not back tearing up the centuries old definition of marriage and the more they hear about these plans the more sceptical about the proposals they become.
“All of the polls commissioned by the major polling companies are published online, plus C4M have sent copies to No 10. Therefore there can be no excuse for this misleading letter that is riddled with errors.”