The world's leading economic powers are moving towards an acceptance that Greece will default on up to half of its €350bn sovereign debts, according to reports from meetings in Washington yesterday. They are believed to be working on concrete plans to deal with these huge losses and their repercussions.

This news – almost regardless of any words of qualification that emerge this weekend – will have a resounding effect on the febrile markets when they open on Monday.

Unconfirmed reports circulated yesterday that G20 leaders have recognised that the Athens government cannot cope with the scale of its debt burden and that there will eventually need to be a considerable reduction in the face value of Greek debt. The Finance Minister, Evangelos Venizelos, was quoted by two Greek newspapers as suggesting that a 50 per cent writedown for the holders of Greek bonds would be the "best option".

The priority for national policymakers now, apparently, is to contain the impact by recapitalising banks and boosting the powers of the European bailout fund by the time of the next G20 meeting in November.

Last night, the Chancellor, George Osborne, said: "There is a recognition here that the global debt crisis has entered a dangerous phase." Asked whether the G20 was preparing for a Greek default, he tried to dampen speculation by saying: "No one has put forward a plan for a Greek default."

The reports that officials are planning for default coincided with a warning from Dr Nouriel Roubini, the economist known as "Dr Doom" since he predicted the 2008 credit meltdown, that unless European leaders beef up the resources of the eurozone bailout fund, Italy and Spain could be forced out of the euro by panicking markets.

The US economist said in an interview: "Italy and Spain are toast, unless we have a tripling or four times as much of official resources to backstop them." In the interview with Emerging Markets magazine, he said that another global downturn is now inevitable and that the only open question is how severe it will be. "At this point the debate is not whether we're going to have a double-dip recession or not. The double dip has started. The only question is whether we are going to have a mild recession in advanced economies or whether we're going to have a severe recession... The answer depends on whether you can keep Italy and Spain."

G20 leaders, meeting in Washington yesterday, pledged to take decisive action to halt the crisis over eurozone sovereign debt, which has triggered turmoil in the financial market after the delay in approving the Greek bailout package agreed in July.

This followed an earlier pledge made by the G20 finance ministers on Friday to "maximise" the impact of the bailout fund, although they gave no specifics.

The US Treasury Secretary, Timothy Geithner, stressed in a BBC interview that the costs of the crisis are growing with each day the eurozone leaders fail to take decisive action. "These things have the classic dynamic that the longer you wait, the harder it is to solve, the more expensive it is to solve. There's a huge premium on early action." But he also claimed that the penny has finally dropped for European politicians. "I believe, on the basis of all my private conversations, that the leadership of Europe are going to move more forcefully and do what's necessary to reverse this erosion of confidence."

His view was echoed earlier in the week by Mr Osborne, who said: "The eurozone is the epicentre of the global problems. That is acknowledged by the eurozone themselves." However, the Chancellor rejected the suggestion that calls from the IMF's head, Christine Lagarde, this week for some countries to "do more" to accommodate growth in the short term should prompt a change of course from Britain in its deficit reduction programme. He said: "I'm very clear that we've got a plan, we're sticking to the plan."

Mr Osborne added that high borrowing levels were driving the current crisis. "It's all rooted in the ability of these countries to deal with their debt problems, a problem that we articulated in Britain 18 months ago. We got ahead of the curve."

The Chancellor went on to say that he is open to the idea of the Bank of England ring-fencing cash for loans to small or medium-sized firms that are in need of help to expand. There is a growing belief that the Bank of England will authorise another emergency injection of cash with a second round of "quantitative easing" – printing money. As much as £300bn may be pumped into the economy.

In a meeting on the sidelines of the IMF, Jean-Claude Trichet, head of the European Central Bank, admitted that the current situation is more precarious than when Lehman Brothers collapsed. There was no longer the belief in the markets that countries such as Greece would not default on their debts, he said.

One of the plans being discussed is for the European Financial Stability Fund (EFSF) to be strengthened, possibly by guaranteeing bigger European Central Bank purchases of Spanish and Italian sovereign debt to try to insulate them from the Greek crisis.

Ms Lagarde also hinted at the idea that both the ECB and the EFSF should buy bonds to show greater resolve to calm the markets.

It was claimed yesterday that the rescue package would come largely from the EFSF, set up last year – of which Britain is not a member. That could mean some respite for British tax-payers, if not British banks.