The former Federal Reserve chairman, Alan Greenspan, has conceded that the global financial crisis has exposed a "mistake" in the free market ideology which guided his 18-year stewardship of US monetary policy.

A long-time cheerleader for deregulation, Greenspan admitted to a congressional committee yesterday that he had been "partially wrong" in his hands-off approach towards the banking industry and that the credit crunch had left him in a state of shocked disbelief. "I have found a flaw," said Greenspan, referring to his economic philosophy. "I don't know how significant or permanent it is. But I have been very distressed by that fact."

It was the first time the man hailed for masterminding the world's longest postwar boom has accepted any culpability for the crisis that has engulfed the global banking system.

During a feisty exchange on Capitol Hill, he told the House oversight committee that he regretted his opposition to regulatory curbs on certain types of financial derivatives which have left banks on Wall Street and in the Square Mile facing billions of dollars worth of liabilities.

"I made a mistake in presuming that the self-interests of organisations, specifically banks and others, were such that they were best capable of protecting their own shareholders and their equity in the firms," said Greenspan.

His remarks came as Lord Myners, the newly appointed City minister, told the Guardian that a new agency was planned to oversee the UK government's £37bn share of high street banks.

As the crisis continued to depress global stockmarkets, the US treasury secretary, Henry Paulson, also offered a partial confession, admitting he ought to have anticipated a meltdown in the US mortgage industry widely blamed for triggering the crisis. "I could have seen the sub-prime crisis coming earlier," he told the New York Times. He added: "I'm not saying I would have done anything differently."

In prepared remarks before the House of Representatives, Greenspan, 82, who retired in 2006, called the financial crisis a "once-in-a-century credit tsunami" and said it had "turned out to be much broader than anything I could have imagined".

He suggested his trust in the responsibility of banks had been misplaced: "Those of us who have looked to the self-interest of lending institutions to protect shareholders' equity (myself especially) are in a state of shocked disbelief."

The congressional committee's Democratic chairman, Henry Waxman, pressed him: "You found that your view of the world, your ideology, was not right, it was not working?" Greenspan agreed: "That's precisely the reason I was shocked because I'd been going for 40 years or so with considerable evidence that it was working exceptionally well."

In an interview with the Guardian, Myners said the new oversight agency would monitor the government stakes in Royal Bank of Scotland, HBOS, and Lloyds TSB.

Myners, who was chairman of Guardian Media Group, which publishes the Guardian, until he took the ministerial post three weeks ago, said the agency would have its own chief executive and board of directors. "The government will hold [the shares] through a separate agency," he said.

But he admitted that the Treasury, the Bank of England and the Financial Services Authority were doing a "lot of preparatory work" in anticipation of further problems in the financial system.

Sorry, the hardest word

Congressman Henry Waxman "My question is simple. Were you wrong?"

Greenspan "Partially ... I made a mistake in presuming that the self-interest of organisations, specifically banks, is such that they were best capable of protecting shareholders and equity in the firms ... I discovered a flaw in the model that I perceived is the critical functioning structure that defines how the world works. I had been going for 40 years with considerable evidence that it was working exceptionally well. The overall view I take of regulation is, I took an oath of office when I became Federal Reserve chairman. I'm here to uphold the laws of the land passed by Congress, not my own predilections."