The House of Commons voted overwhelmingly on Tuesday to approve a bill legalizing same-sex marriage in Britain, indicating that the bill is assured of passage as it moves through further legislative stages.
But in a major setback for Prime Minister David Cameron, who championed the bill, it appeared that more than half of the lawmakers in his Conservative Party voted against the measure or abstained.
After a six-hour debate, the Commons vote was 400 to 175 for the bill. It will have to pass in the House of Lords, where delaying tactics by opponents are possible, but Mr. Cameron has said that he plans to have the bill enacted into law sometime this summer.
Although 132 of the 303 Conservative lawmakers voted for the bill, early calculations by opponents within the party were that as many as 170 or 180 others in the party had broken with the prime minister, 140 by voting against the bill and as many as 40 others by abstaining.
In modern times, few prime ministers have faced such an extensive rebellion in their own ranks, and the outcome seemed likely to add to the growing ferment among backbench Conservatives about Mr. Cameron’s leadership on a wide range of issues, including Britain’s shrinking defense budget and its increasingly uneasy ties with the European Union.
The divisions over same-sex marriage have been less vehement in Britain than they have been in France, where a similar bill backed by President François Hollande has prompted rival demonstrations in Paris recently that have drawn tens of thousands into the streets. Discussions in the French Parliament have been equally impassioned, where a marathon debate on the issue, now in its second week, has featured angry insults across the floor of the National Assembly and more than 5,300 amendments.
By comparison, the debate in the House of Commons was mostly understated, with a strong undercurrent of realism among lawmakers who oppose gay marriage but sense that the battle is already lost, not only in the crushing parliamentary majority favoring change but in a wide variety of opinion polls that have shown strong public support for the measure put forward by Mr. Cameron.
Nonetheless, some Conservative lawmakers added a strident note to Tuesday’s debate. Sir Roger Gale, a right-wing backbencher, pulled no punches. “It is not possible to redefine marriage,” he said. “Marriage is the union between a man and a woman, has been historically, remains so. It is Alice in Wonderland territory, Orwellian almost, for any government of any political persuasion to seek to come along and try to rewrite the lexicon. It will not do”.
Another discomfited backbencher, Edward Leigh, struck a note that carried a warning for Mr. Cameron, who spoke openly of his support for gay marriage almost as soon as he became the Conservative leader in 2005. He is said to have adopted the position under the strong influence of his wife, Samantha, a creative adviser to a London fashion accessories company — and to have made it part of a broader push to reposition the Conservatives and make them more appealing to younger and less traditional voters.
But to Mr. Leigh, and for many others in the party’s parliamentary bloc, pushing the bill was a step too far. “We should be in the business of protecting cherished institutions and our cultural heritage; otherwise what, I ask, is a Conservative Party for?” he said.
He added, “We are alienating people who have voted for us all of thir lives, leaving them with no one to vote for.”
Mr. Cameron is trying to modernize the Conservatives, and to position the party for 2015, when it will have to battle against a resurgent Labour Party riding high in the polls and strongly in favor of same-sex marriage, as is the third major party in British politics, the Liberal Democrats.
Although Mr. Cameron played the role of protagonist for the bill before British lawmakers, and risked adding momentum to a restiveness about his leadership among right-wing Conservative backbenchers, he was not among the 70 or so members of the House of Commons who spoke in the debate on Tuesday.
But he addressed reporters earlier at 10 Downing Street, where he had been holding talks with Vice President Joseph R. Biden Jr.
“Today is an important day,” he said, before heading for the Commons to cast his vote. “I am a strong believer in marriage. It helps people commit to each other, and I think it is right that gay people should be able to get married too.”
“Yes, this is about equality,” he added. “But it is also about making our society stronger. I know there are strong views on both sides of the argument; I accept that. But I think this is an important step forward for our country.”
A day after the newly confirmed archbishop of Canterbury, Justin Welby, took office saying that he shared the Church of England’s opposition to marriage between people of the same gender, three cabinet officials said in a letter published in The Daily Telegraph that the new legislation was “the right thing to do at the right time.”
“Marriage has evolved over time,” the letter said. “We believe that opening it up to same-sex couples will strengthen, not weaken, the institution.”
It continued: “Attitudes toward gay people have changed. A substantial majority of the public now favor allowing same-sex couples to marry, and support has increased rapidly.”
The three ministers — George Osborne, the chancellor of the Exchequer, Foreign Secretary William Hague and Home Secretary Theresa May — also asked whether it was “any longer acceptable to exclude people from marriage simply because they love someone of the same sex.”
The debate divided Britain’s Conservatives, who lead in uneasy coalition with the Liberal Democrats.
Opponents of the legislation argued that it would alienate traditional Conservative voters, jeopardizing Mr. Cameron’s prospects in the 2015 national election. But supporters said it would bring in new backing from outside the party.
Archbishop Welby, 57, was confirmed Monday to replace the Most Rev. Rowan Williams, who has retired after 10 years in office.
The new archbishop, the spiritual head of the world’s 77 million Anglicans, endorsed the traditional view that while the Church of England has no objection to civil partnerships between people of the same gender, it is, as a recent church statement put it, “committed to the traditional understanding of the institution of marriage as being between one man and one woman.”
Ed Miliband, the leader of the opposition Labour Party, said Monday that he would be “voting for equal marriage in the House of Commons, and I’ll be doing so proudly.” He also said he would urge his 255 legislators in the 649-member body to vote with him, although a small group will probably not.
“I’ll be voting for equal marriage for a very simple reason: I don’t think that the person you love should determine the rights you have,” Mr. Miliband said Monday.
The legislation, which applies to England and Wales, would permit civil marriage between same-sex couples, but specifically exempt the Church of England and other faiths from an obligation to perform such ceremonies. Some faith groups, including the Quakers, have said they want the legal right to perform same-sex marriages.
In their letter, Mr. Osborne, Mr. Hague and Ms. May said: “Our party also has a strong belief in religious freedom, a vital element of a free society. The bill ensures that no faith group will be forced to conduct same-sex marriages. The legal advice is clear that these protections for religious groups cannot be overturned by the courts.”
It said: “Religious freedom works both ways. Why should faith groups, such as the Quakers, that wish to conduct gay marriages be forbidden from doing so? This bill will enhance religious freedom, not restrict it.”