The British House of Lords voted Monday to approve a same-sex marriage bill, clearing the way for the measure to become law. The bill will now face a final vote by the lower House of Commons, where it is also expected to pass.
The plan to introduce gay marriage has pitted Cameron against many in his own party. Activists say it is driving Conservative voters toward the U.K. Independence Party, which made gains in local elections in May at the expense of the Tories and their Liberal Democrat coalition partners.
The prime minister was only able to get the legislation through the Commons with the support of the opposition Labour Party.
Under current law, gay couples in the country can only enter into civil partnerships.