Alistair Darling has already spent almost a fifth of Britain's GDP on bailing out its shattered banking system – more than any other major economy, according to a grave assessment of the world financial crisis published today by the International Monetary Fund.

With G20 finance ministers due to gather in Sussex next Friday for a two-day meeting before the London summit in April, the IMF has totted up the costs of financial bailouts so far. It calculates that the UK has spent as much as 19.8% of its GDP, topping the table of G20 countries.

The US, where the investment bank Bear Stearns and the insurer AIG have both been rescued with public finds, has spent just 6.8% of its GDP. Only Norway has come close to the UK, spending 13.8%.

Opposition politicians leapt on the figures as evidence of the economic damage inflicted by the credit crunch. Philip Hammond, the shadow chief secretary to the Treasury, said: "This is a stark illustration of the true cost of Labour's Age of Irresponsibility. Thanks to the failings of the banking regulatory framework that Gordon Brown put in place, we have spent more than twice as much on bailing out the banking system as the United States – and taxpayers are on the hook for billions of pounds as a result."

Vince Cable, the Liberal Democrat Treasury spokesman, said: "This is a direct consequence of playing host to international banks. Britain is in an extremely exposed position. These are global banks, but they're not being rescued by the globe."

He argued that instead of focusing on international action on bonuses at the G20 summit, Gordon Brown should be pushing for a forced separation between risky investment banking, and the staid deposit-taking institutions that are essential to the economy. "We have to separate deposits from the very risky global casino."

A Treasury spokesman said: "The UK is leading the world in taking action to clean up banks' balance sheets and provide them with greater confidence to increase lending in the economy. As the chancellor has said, the alternative is a failure of the banking system, here and elsewhere, which will make the recession longer and more painful, putting more jobs at risk."

He added that the final cost might be lower if the value of assets owned by the part-nationalised banks bounced back.

In a series of discussion papers, the IMF draws an alarming picture of the trail of policy mistakes and complacency that led to the credit crunch, saying: "At the root of market failure was optimism bred by a long period of high growth, low real interest rates and volatility." The crucial lesson world leaders must now learn, it says, is that, "flawed incentives and interconnections in modern financial systems can have huge macro-economic consequences".

It calls for more coherent regulation of financial products, including non-bank institutions such as hedge funds; and better co-ordination between international organisations at the heart of the financial system, including the IMF itself.

Its calculations date from 18 February, before the United States' latest bailout for Citigroup and the Treasury's asset-protection scheme for the Royal Bank of Scotland.