The moment the vote result was announced in the House of Commons
MPs have backed government plans for £12bn in welfare cuts amid a Labour revolt over its leadership's call for its MPs not to oppose the changes.
The Commons backed the Welfare Reform and Work Bill at Second Reading by 308 to 124 votes.
But scores of Labour MPs defied orders to abstain and instead voted against the bill, which includes plans to limit child tax credit to two children.
Forty-eight MPs are thought to have defied the leadership over the issue.
The LabourList website said those who rebelled included leadership contender Jeremy Corbyn and London mayoral candidates Sadiq Khan and David Lammy.
During a five-hour debate one MP, John McDonnell, said he would "swim through vomit" to oppose the legislation.
However, a Labour amendment seeking to derail the legislation was also defeated by 308 votes to 208.
Acting Labour leader Harriet Harman has faced criticism for her stance on the issue, with many MPs saying she should have been more outspoken in her opposition to curbs on child tax credits and cuts to other in-work benefits.
The scale of the Labour rebellion will not be known until details of the Commons divisions are published later.
Mr Duncan Smith said welfare would be put on a "sustainable footing".
However, there are reports that just under 50 MPs may have defied the leadership, more than 20% of the parliamentary party. Among those expected to vote against was leadership contender Jeremy Corbyn.
His three rivals, Andy Burnham, Yvette Cooper and Liz Kendall, are thought to have followed Ms Harman's lead and abstained.
Reacting to the vote, Labour MP Diane Abbott tweeted: "Just voted against Tory welfare bill. Sorry for colleagues who knew it was wrong but abstained. We weren't sent to Parliament to abstain."
The bill, which also seeks to lower the overall household benefit cap from to £20,000 (and £23,000) in London and to train a further three million apprentices, has now cleared its first parliamentary hurdle and will now move onto more detailed scrutiny.
In a passionate debate, Conservative MPs lined up to support the measures while many Labour MPs said they could not support the bill, which was also opposed by the SNP, the Lib Dems, Plaid Cymru and the Greens.
Mr McDonnell said: "I would swim through vomit to vote against this Bill and listening to some of the nauseating speeches tonight I think we might have to.
"Poverty in my constituency is not a lifestyle choice, it is imposed upon people.
"We hear lots about how high the welfare bill is, let's understand why that's the case.
"The housing benefit bill is so high because for generations we've failed to build council houses, we've failed to control rents, we've done nothing about the 300,000 properties that stand empty in this country."
Work and Pensions Secretary Iain Duncan Smith said Labour was beset by "internal fear and loathing" and that the bill would put welfare funding on a "more sustainable footing" while protecting those most in need.
Speaking after the vote, he said: "Nearly 50 Labour MPs have defied their leadership and opposed our welfare reforms which will move our country from a low wage, high tax and high welfare economy to a higher wage, lower tax and lower welfare society.
"It's clear that Labour are still the same old anti-worker party - just offering more welfare, more borrowing and more taxes."