Labour leader Ed Miliband branded David Cameron an "absolute disgrace" in an ill-tempered Prime Minister's Questions clash over welfare reform.
Mr Miliband claimed the proposed changes would result in cancer patients losing £94 a week in benefits and urged Mr Cameron to halt the reforms.
Mr Cameron accused the Labour leader of using the issue as a "smokescreen" to cover his party's reluctance to reform the welfare system. He told Mr Miliband, who has suffered a bruising week, it was "weak leadership of a divided party".
Mr Miliband shot back: "What an absolute disgrace, to describe talking about cancer patients in this country as a smokescreen."
Earlier, Mr Cameron had insisted that plans in the Welfare Reform Bill to cap benefits at the average level of earnings were correct. The legislation is set to clear the Commons on Wednesday night but there had been reports of a U-turn on the £26,000 cap.
He told Tory Margot James (Stourbridge): "We are right to reform welfare. Welfare costs have got out of control in our country."
Mr Miliband was jeered by Tory MPs as he rose to ask his first question at the despatch box. The Labour leader asked: "When the Prime Minister signed off his welfare Bill, did he know that it would make 7,000 cancer patients worse off by as much as £94 a week?"
Mr Cameron said that was "simply not the case" and told him: "We are using exactly the same definition of people who are suffering and are terminally ill as the last government. We want to make sure those people are helped and protected."
He told Mr Miliband: "If you are in favour of welfare reform, you want to encourage people to do the right thing, it's no good talking about it, you've got to vote for it."
Mr Miliband said Macmillan Cancer Support had raised concerns about how the Bill's measures would affect patients. "These are people who have worked hard all their lives, who have done the right thing, who have paid their taxes and when they are in need, the Prime Minister is taking money away from them," he said.