David Cameron has ended a unilateral truce on the global financial crisis with the government, accusing Gordon Brown's policies of 'fundamentally failing'.
Decisive action that saw Britain's banks bailed out and then part-nationalised had seen public perception of Labour and the prime minister improve.
The Conservative party, fearful of attempting to gain political capital by opposing the government's actions, had spent the last month largely silent on the economy.
But in a speech in the City this morning, Mr Cameron said Mr Brown had built up the British economy "on debt".
He said that just 12 months ago the prime minister had said the City was entering a "golden age", a far cry from his current line of investors living in an "age of irresponsibility".
"The economic assumptions that Gordon Brown made in the last decade now lie in ruins," Mr Cameron said.
"The complete and utter failure of their economic record has never been more clear to see."
The speech marks a fundamental shift in Tory strategy. Since their party conference in Birmingham was overshadowed by global financial collapse, the party has pursued a policy of critical support for the government.
But today, Mr Cameron tried to associate Mr Brown's record in government with the irresponsible risk taking of City bankers.
"Over the past decade, we have seen a total breakdown of economic responsibility," Mr Cameron said.
"From the government, as it has spent and borrowed without restraint. And from our financial sector, which has taken decisions which have harmed the rest of our economy."
Mr Cameron argued for a more varied and balanced economy, with over-reliance on the City replaced by a wider range of industries.
He also defended the Conservative philosophy in the face of global market failure.
"Critics assert that the centre-right has no solution and the change we need is a turn to the left," he said.
"They are wrong. My argument today is that the change we need is the modern Conservatism that I have been advocating."
The Conservatives have suffered a drop in the polls since the crisis began, with a steady Labour improvement during the same period.
Private briefings earlier this week indicated party strategists expect the lead to be reduced to single figures.