The prime minister has rejected David Cameron's calls for a general election, insisting it would lead to "chaos".
But Gordon Brown's statement was mocked by the Conservative leader, who demanded a "better answer" for why the public should be denied a vote.
The Liberal Democrat leader also backed calls for an election, but focused on the need for wider political reform following the expenses row.
Following the prime minister's warning that a Tory government would cause chaos, Cameron said that was an admission of a looming Labour defeat.
"So there we have it, the first admission that he thinks he's going to lose," said the Conservative leader.
He added that an "unelected prime minister" was not the answer to Britain's problems.
"We've got to give the public their voice and the country the chance of a fresh start," he insisted.
Brown responded that the House should have "some humility about what has happened in the last few days".
He said there was a "duty to sort the problem out", and while progress has been made there is "a lot of work still to do".
Rejecting calls for an election, he said the government was delivering help for families and businesses.
A Conservative government would cut schemes such as the New Deal and Sure Start, he warned.
But Cameron retorted: "The people will just hear the arrogance of a prime minister who won't let the public decide."
"The prime minister calls elections 'chaos'. I call them change. Why can't we have one?" he asked.
Highlighting Tory plans to cut spending, Brown told the Commons: "Yes, there would be chaos with public spending cuts under the Conservatives."
Nick Clegg, meanwhile, insisted there was a "once in a generation chance to change politics for good".
The Lib Dem leader said expenses were "just the tip of the iceberg" and added that "the whole way we do politics must now be transformed".
Calling for more transparency and electoral reform, he added that the first-past-the-post system would always "breed arrogance and secrecy".
Brown said that both Parliament and the executive should be made more accountable.
And there should be a wider debate with the public on such issues, he added.
Dods Parliamentary Communications Ltd