George Osborne is facing anger from Tory backbenchers and Labour after announcing Britain is to commit another £10 billion to the IMF.
The Chancellor insisted the increased funding was vital to protect jobs and growth in this country.
But critics said he was putting the taxpayer on the hook for more bailouts of the struggling eurozone.
He was also accused of dodging a potentially embarrassing parliamentary vote on the extra contribution.
Finance ministers and central bank governors struck the deal, which should boost the IMF's resources by US dollars 430 billion, at a meeting in Washington.
Alongside the UK's increase, Japan is to contribute an extra 60 billion dollars, South Korea 15 billion dollars, Switzerland 10 billion dollars and Australia seven billion dollars.
However, the US and Canada have refused to add any more into the pot.
Osborne said: "The UK sees itself as part of solution to the challenges facing the global economy, not part of problem. We are helping to solve the global debt problem rather than adding to it.
"Jobs and growth in Britain depend on stable world economy. That needs a strong IMF.
"And because we have taken strong action to rescue our own economy, we can be one of many countries that can support the IMF, instead of being bailed out by the IMF."
Parliament has previously approved around £40 billion in support for the IMF, of which about £30 billion has already been committed.
If the increase had gone beyond the £10 billion "headroom" still available to Mr Osborne it would have required a fresh vote by MPs.
Committing money does not mean it will necessarily be drawn against and, because it would be given in the form of a loan, it would not deplete public spending budgets.
Shadow chancellor Ed Balls accused Osborne of signing up to a "sticking plaster" deal and "running scared" of parliamentary scrutiny.
Rupert Murdoch also added his voice to the criticism, tweeting Osborne must be "mad":
And Conservative Peter Bone branded the move "bonkers" and said money used to prop up the euro would be wasted.
"It seems to me it is all about bailing out the eurozone," he said. "It should not be up to British taxpayers to shore up a doomed project that is for the benefit of our European colleagues.
"People will not understand how we can have all these cuts but put £10 billion at risk for other countries. It is bonkers."
However, Bone said there was now no obvious way of forcing a Commons vote on the issue.
"It does seem very strange that £10 billion can be spent without getting a proper parliamentary debate," he added.
Tory MP Mark Pritchard, secretary of the influential 1922 Committee, said Osborne appeared to have "got away with the politics" of the issue by avoiding a fresh vote.
But he told BBC Radio 4's PM programme the UK should not be underwriting a currency that "clearly is not working".
"Indirectly that is exactly what British taxpayers' money and IMF funding is going to do," he added.
However, other senior Conservative figures expressed support for Osborne.
Treasury Select Committee chairman Andrew Tyrie said the increase was "essential".
"The IMF is the only fire-brigade available to the global economy," he said. "It is vital that the IMF has the necessary tools to deal with the current eurozone crisis and the risks to wider global financial stability.
"Any IMF loans to the eurozone must be on rigorous terms, with full conditionality.
"The IMF must not flinch from its long-standing policy of negotiating only with member countries."
The Tory MP added: "Britain benefits more than most from having a tough global watchdog and no country outside the eurozone has more reason to want the crisis resolved than the UK."
Conservative backbencher Laura Sandys hit out at Balls for "shallow opportunism".
"He isn't taken seriously at home and now with this opportunistic approach he isn't taken seriously around the world," she said.
"George Osborne is taking the tough, responsible decisions. It's about time Labour started doing the same."
Camborne and Redruth MP George Eustice said: "Britain should play its part to help stabilise the world economy so we should support the IMF but we must also ensure that the financial help only goes to support countries and that eurozone members take responsibility for their currency."
Tory MP Douglas Carswell branded the decision "madness" and "IMF bailout bull".
Writing on his blog, he suggested the loan "makes you wonder how all those earlier bailouts have worked out".
Osborne told BBC Radio 4's Today programme: "When you look at the Tory backbench opinion, of course some people have been critical and they have been critical all along, but actually quite a lot of Conservative backbenchers have come out and said this is the necessary and right thing.
"I think there is a broad body of Conservative opinion that understands we are doing the right thing."
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