The British government is making urgent preparations to cope with the fallout of a possible Greek exit from the single currency, after the governor of the Bank of England, Sir Mervyn King, warned that Europe was "tearing itself apart".

Reports from Athens that massive sums of money were being spirited out of the country intensified concern in London about the impact of a splintering of the eurozone on a UK economy that is stuck in double-dip recession. One estimate put the cost to the EU of Greece making a disorderly exit from the currency at $1tn, 5% of output.

Officials in the United States are also nervously watching the growing crisis: Barack Obama on Wednesday described it as a "headwind" that could threaten the fragile American recovery.

In a speech in Manchester before flying to the United States for a summit of G8 leaders, the British prime minister, David Cameron, will say the eurozone "either has to make up or it is looking at a potential breakup", adding that the choice for Europe's leaders cannot be long delayed.

"Either Europe has a committed, stable, successful eurozone with an effective firewall, well capitalised and regulated banks, a system of fiscal burden sharing, and supportive monetary policy across the eurozone, or we are in uncharted territory which carries huge risks for everybody.

"Whichever path is chosen, I am prepared to do whatever is necessary to protect this country and secure our economy and financial system."

Officials from the Bank, the Treasury and the Financial Services Authority are drawing up plans in the expectation that a Greek departure from monetary union – increasingly seen as inevitable by financial markets – could be as damaging to the global economy as the collapse of Lehman Brothers in September 2008.

With a second election in Greece called for 17 June, King dropped a strong hint that the Bank would take fresh steps to stimulate growth if policymakers in Europe failed to deal with the sovereign debt crisis.

"We have been through a big global financial crisis, the biggest downturn in world output since the 1930s, the biggest banking crisis in this country's history, the biggest fiscal deficit in our peacetime history and our biggest trading partner, the euro area, is tearing itself apart without any obvious solution," he said.

Doug McWilliams, of the Centre for Economic and Business Research, said a planned breakup of the single currency would cost 2% of EU GDP ($300bn) but a disorderly collapse would result in a 5% drop in output, a $1tn loss. "The end of the euro in its current form is a certainty," he added.

Alistair Darling, who was Chancellor of the Exchequer under the former Labour administration, said: "This has the seeds of something disastrous. It is madness. If it spreads to bigger countries, this could be really disastrous for Europe. It could consign us to years of stagnation."

Capital flight from Greece has increased since it became clear that a coalition government could not be formed after the election earlier this month. The Greek president, Karolos Papoulias, said citizens were withdrawing their money amid "great fear that could develop into panic" at the risk of a debt default and exit from the euro area, according to minutes of their meetings posted on the presidency's website. In little more than a week following the election on 6 May, €3bn was withdrawn from bank accounts. The central bank reported that €800m was taken out in a single day earlier this week.

The head of the International Institute of Finance banking lobby, Charles Dallara, said money was leaving Greece at a growing pace due to political uncertainty. "There has been a pickup of deposit flight from Greece, but I think that is stabilisable once you get a new government in place, if that government reaffirms its intention to remain in the eurozone." The damage to the rest of Europe if Greece were to leave the euro would be "somewhere between catastrophic and armageddon", he said.

The Spanish prime minister, Mariano Rajoy, told parliament that his country faced trouble financing itself as borrowing costs shoot up to "astronomic" levels. The Irish finance minister, Michael Noonan, said Dublin's plan to return to capital markets in late 2013 might not be achievable because of the uncertainty.

The first meeting between French president François Hollande and German chancellor Angela Merkel helped to calm nerves in the markets at one stage, with suggestions that Berlin might be amenable to initiatives to boost growth in Greece and the other austerity-stricken nations of the eurozone.

But the jittery mood was underlined by a fall in European shares and the single currency late in the day amid reports that the European Central Bank was cutting off its funding lifeline to Greek banks that had failed to amass enough capital to protect them from future losses.

The ECB later said it expected the Greek central bank to use part of the €130bn bailout from the EU and IMF to ensure that the country's banks were safeguarded from collapse, and that they would receive additional help from Frankfurt only once this had happened. Already delayed by the political uncertainty in Greece, €18bn is now expected to be released to recapitalise the banks.

Sony Kapoor, of the Brussels-based Re-Define thinktank, said: "The high-stakes game of chicken between Greek and other EU politicians must end now. Those saying that a Greek exit from the eurozone will not be a big deal either don't know what they are talking about, or have some ulterior motives. The social, political and economic damage to the EU from a Greek exit is potentially incalculable."

At the G8 summit, which starts on Friday, Obama will press Merkel to lean more towards a growth package for Europe, instead of pressing so hard for the austerity measures that were rejected by Greek voters.

But foreign affairs analysts said that Obama's leverage with the European leaders is minimal. Although the US has the economic muscle to help Europe out of its mess, the Obama administration has taken the strategic decision not to become involved directly.

Instead, Obama is to use the Camp David summit for some quiet diplomacy, hoping to sway Merkel to endorse some immediate actions to help growth.

King, speaking at the publication of the Bank of England's quarterly inflation report, said growth in Britain was weaker and inflation higher than Threadneedle Street had expected three months ago. It would take until 2014 for output to return to where it was in 2008, when Britain's deepest post-war recession began.

"What is so depressing about it is that this is a rerun of the debates in 2007/08 – these are not liquidity problems, they are solvency problems," King said. "Imbalances between countries in the euro area have created creditors and debtors and at some point the credit losses will need to be recognised and absorbed and shared around," he said.

"Until that is done, there will not be a resolution. That is why just kicking the can down the road is not an answer. The European Central Bank has performed heroically in trying to buy time but that time hasn't been used to put in place fundamental underlying solutions."

• This article was amended on 23 May 2012. The orginal said a estimate put the cost of a disorderly collapse of the euro at 5% of eurozone GDP, about $1tn. That has been corrected to 5% of EU output.