Opponents of same-sex marriage in the country said the petition shows widespread support for the traditional definition of marriage as the union of a man and a woman, The Daily Telegraph reported.
Civil partnerships have been legal in Britain since 2005.
The government's proposal this month would redefine civil marriage laws so the term can apply to civil unions between same-sex couples.
The Coalition 4 Marriage, which started the petition, has urged the government to keep the definition of marriage as the voluntary union between one man and one woman for life.
"There has been a staggering response to the petition, launched last month, which shows just how many ordinary men and women care about this issue," said Colin Hart, the director of the petition campaign.
"I hope the government will consider the growing opposition to its proposals, which are being pushed without the British people being given an opportunity to make their views clear."
Senior Conservatives, Christian leaders, the country's largest Muslim group and the head of Britain's Sikh organizations expressed opposition to the same-sex marriage proposal.
Cameron said Conservatives should support same-sex marriage because it would encourage stability and commitment in relationships.
A rival petition campaign supporting gay marriage, the Coalition 4 Equal Marriage, has received 33,400 signatures.
"Equality shouldn't be just mob rule," said Conor Marron, who started the petition with his partner. "There are things that are fair and right and should be done. The vast majority of people either support what we believe or don't care very much."