The politicians warned that the ruling party will lose significant numbers of voters in the 2015 election if the plans enter law.
Lawmakers are due to get their first vote Tuesday on the proposed bill, which would enable same-sex couples to get married in both civil and religious ceremonies. It would also allow couples who had entered into civil partnerships to convert their relationship to a marriage.
More than 20 heads of local Conservative associations delivered a letter urging a delay in the vote to Downing Street on Sunday.
Cameron supports the bill, but the issue has divided his Conservative Party.
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