Chancellor George Osborne "capitulated" when he committed a further £10bn to the International Monetary Fund, Ed Balls has said.
The shadow chancellor argued that Mr Osborne was "conspiring in allowing the IMF to become the de facto central bank of the euro area" by agreeing to the deal at a recent meeting in Washington.
In a statement to MPs on 23 April 2012 the chancellor insisted it was in Britain's national interest to provide the extra funding, which falls below the threshold which would trigger a Commons vote.
But Mr Balls countered: "Allowing euro leaders room for further delay and exposing the IMF and British taxpayers' money when rich eurozone countries will not act, is categorically not in the British national interest."
He added that if Mr Osborne had decided that the UK should contribute "a fraction more" he would have had to ask for parliamentary approval, predicting that the government would have had difficulty persuading MPs to vote in favour of such a move.
The Labour spokesman asked: "Isn't this chancellor running scared of both sides of this House of Commons?"
Mr Osborne also faced criticism from MPs on his own benches anxious to prevent UK taxpayers' money being spent on propping up the euro.
Tory backbencher Douglas Carswell quoted Mr Osborne telling MPs last year that Britain would not contribute money to bailout the eurozone "either directly or through the IMF".
Mr Carswell demanded to know: "Has he changed his mind or was he playing with words?"
The chancellor replied: "We are not contributing to the eurozone bailout funds."
He added: "We are not contributing money to the IMF that can be put into those funds, that is something we've also insisted on."
The IMF would support countries, but not currencies, Mr Osborne said.
The international institution says it has received firm commitments of more than $430bn from a number of countries.
The money doubles the fund's firepower, which was in danger of becoming overwhelmed by the eurozone crisis.
It can lend that money to other countries in financial stress, with the aim of creating global financial stability and a sounder footing for the world economic recovery.