US Edition The extra commitment will take Britain’s total contributions to the International Monetary Fund (IMF) to £40bn, putting every household on the line for £1,600.
The Chancellor struck the deal in Washington as part of a broad agreement between a number of non-eurozone countries to pump an extra $200bn into the IMF’s resources to help underpin the global recovery.
Alongside the UK, the Republic of Korea pledged a further $15bn (£10bn), Australia $7bn , and Singapore $4bn.
Sources said Russia has agreed about $10bn of extra contributions, with China, India and Brazil still in negotiations but close to a deal.
Japan has committed $60bn and Switzerland about $26bn. In total, the IMF has secured about $400bn in extra funding from members, half of which was pledged by the eurozone countries last December.
However, Tharman Shanmugaratnam, chairman of the International Monetary and Financial Committee at the IMF, said the new funds may not be enough. He warned: “If [the eurozone’s] reforms lose credibility, lose momentum, then - quite frankly - the firewall is not enough.”
Including the eurozone’s €800bn rescue fund, the global “firewall” is about $1.8 trillion – close to the $2 trillion “big bazooka” the markets have been demanding.
But Mr Shanmugaratnam said the firewall, while essential, "by itself is not sufficient".
"The real solution has to do with the fiscal and structural reforms that address the real causes of this crisis, particularly in Europe," he said. “If we didn’t have this firewall in place we wouldn’t have the confidence to embark wholeheartedly on these reforms."
The US and Canada have refused to take part - despite heavy pressure from the IMF managing director Christine Lagarde - on the grounds that the Europeans have not done enough by only increasing their firewall from $500bn to $800bn.
Unveiling the agreement, the Chancellor said: “Because we’ve taken tough action to rescue our own economy we can be one of the countries that can support the IMF.
"The UK sees itself as part of the solution to the global debt crisis rather than adding to it. We will be part of the global effort.”
He added: “It is a loan with interest, not a gift. Let me reiterate, it does not add to our deficit or national debt. It is not money that could otherwise be spent on public services, and no country has ever lost any money lending to the IMF.”
The commitment is sure to stir a backbench rebellion among Conservative MPs as well as opposition from Labour by leaving the UK more exposed than ever to another eurozone rescue.
The Chancellor has repeatedly stressed that the UK would not contribute directly to another eurozone rescue, but there are growing concerns the IMF will have to intervene to bail-out Spain within months.
If the crisis extends to Italy as well, the IMF faces the prospect of lending hundreds of billions of pounds to the crisis-hit region.
Of the UK’s existing £30bn IMF exposure, £5.5bn has already been committed on various programmes, including those in Greece, Portugal and Ireland. Added to which, a further £10bn is exposed through the European Union rescue fund and a one-off bi-lateral loan to Ireland.
Anticipating objections, the Chancellor said the additional contribution was “broadly the same as” the extra commitments made by Labour at the height of the financial crisis in 2009. He added that, excluding the eurozone, the extra £10bn commitment was “broadly in line with our quota size”.
The UK accounts for 4.5pc of the 188-member IMF.
Controversially, though, the Government will not have to put the extra funding to a Parliamentary vote because headroom was built into the last agreement. That 2009 deal, which was formally approved by MPs last year, was opposed by 32 backbench Conservative MPs and Labour - even though it was originally struck by Gordon Brown.
He said: “I’m sure it will be actively discussed in Parliament, but it is also a fact that we have the Parliamentary headroom for this contribution.”
Mr Osborne said the funds would only be released under strict IMF conditions. He added that the deal complied with his earlier requirements – that the eurozone had demonstrated it had taken measures to shore up the economy first, and that other leading nations were part of the deal.
About the eurozone, which recently raised the size of its own bail-out fund from €500bn to €800bn, he said: “They have done a lot in the past year. They have now increased the firewall. They have delivered, and the IMF is right therefore to step in with these additional contributions.”
A rescue for Spain, and possibly Italy, would outstrip anything the IMF has done before, putting the fund at greater risk of making a loss than at any time in its 68-year history.
Mr Osborne insisted, though, that the funds would not be used to prop up currencies, and that any country which received a bail-out would have to meet the IMF's strict conditions.