(Removes reference to Brazil cutting borrowing costs, paragraph four)
* ECB proposes 50 bln euros in monthly bond-buys
* Length of money-printing scheme may be flexible
* Bond-buy plan would aim to buoy economy, inflation
By Paul Carrel and John O'Donnell
FRANKFURT, Jan 22 (Reuters) - The European Central Bank is poised to announce a plan on Thursday to buy government bonds, resorting to its last big policy option for breathing life into the flagging euro zone economy.
Market expectations are sky-high for the ECB to unveil large-scale quantitative easing (QE) - printing money to buy government bonds - despite opposition from Germany's Bundesbank and concerns in Berlin that this could allow spendthrift countries to slacken economic reforms.
The momentous step - which comes as global economic prospects dim - has already prompted the Swiss central bank to abandon its cap on the franc, while Denmark, whose currency is pegged to the euro, was forced to cut interest rates in anticipation of the flood of money.
Canada has cut the cost of borrowing while two British rate setters at the Bank of England have dropped calls for tighter monetary policy.
A euro zone source has said that the ECB's Executive Board has proposed that it buy 50 billion euros ($58 billion) in bonds per month from March.
The broader, 25-member policymaking Governing Council began meeting at 0800 GMT on Thursday to discuss the proposal. ECB President Mario Draghi holds a news conference at 1330 GMT.
"I expect they will deliver, and launch a QE programme that will be probably larger than 500 billion (euros)," said Sassan Ghahramani, CEO of New York-based SGH Macro Advisors, which advises hedge funds.
There is uncertainty, however, about the length of the programme. While some media predicted that it would run until the end of next year, it could possibly be cut short or extended depending on whether or not it is having an impact on the euro zone economy.
The duration is significant. A programme starting in March and running for a year would total about 600 billion euros, based on a purchase rate of 50 billion per month. If a similar plan ran until the end of 2016, it could surpass 1 trillion euros.
Money market traders polled by Reuters expected a 600-billion-euro bond-buying plan.
Euro zone inflation turned negative last month; consumer prices fell 0.2 percent, far below the ECB's target that they should rise just under 2 percent annually.
But there are doubts, and not only in Germany, over whether printing fresh money will work.
"It is a mistake to suppose that QE is a panacea in Europe or that it will be sufficient," U.S. Treasury Secretary Larry Summers said at the World Economic Forum in Davos on Thursday.
"There is every reason to expect that QE will be less impactful in a context like the present one in Europe than it was in the context of the United States."
Scepticism runs deep among Germans and their Dutch neighbours, who fear that it will see their strong economic standing used to sponsor weaker southern states such as Portugal with cheap finance via the ECB.
Earlier this week, tensions boiled over in a debate at the Dutch parliament, where a majority of political parties said they opposed quantitative easing if it would "lead to an increased risk of redistributing financial risks between euro member states".
The ECB has already cut interest rates to record lows, begun buying debt and funnelled hundreds of billions of euros in cheap loans to banks, in the hope that they would lend the money on into the economy and stimulate growth.
Now its last remaining major option is QE, a policy that the U.S. Federal Reserve, Bank of Japan and Bank of England have already used to revive growth since the global financial crisis. Euro zone policymakers are keen to prevent the kind of deflationary spiral which has plagued Japan for years, resulting in weak growth punctuated by recessions.
By buying sovereign bonds, the ECB would show its commitment to pushing up inflation - while also generating a "portfolio effect" under which investors snap up other debt and securities, some outside the euro zone, thereby depressing the euro.
Expectations that the ECB will opt for QE are high, with the euro diving against the dollar in anticipation.
Even French President Francois Hollande jumped the gun on Monday, predicting that the ECB "will on Thursday take the decision to buy sovereign debt".
Germany is worried about the announcement, which will come just three days before an election in Greece where anti-bailout opposition party Syriza is on track to gain roughly a third of the vote.
Highlighting her concerns, German Chancellor Angela Merkel repeated on the eve of the meeting that any move by the ECB to buy government bonds with new money should not be used as an excuse to put economic reforms on the back burner.
Greece's finance minister urged the ECB not to exclude it from QE: "No country needs quantitative easing as much as Greece," Gikas Hardouvelis told German business daily Handelsblatt.
Against this backdrop, Draghi must balance the intense market pressure to act and a need to buoy inflation on the one hand, with a desire to minimise German dissent on the other.
One option to achieve a compromise is for the euro zone's national central banks to bear the brunt of the risk of bond purchases, rather than the ECB itself.
Ireland's finance minister said on Monday such a ploy would make a QE plan "ineffective", yet this scenario may nonetheless be part of the final plan, sources have told Reuters.
RBS economist Richard Barwell said this would contradict the concept of solidarity among euro zone countries. "It breaks the d'Artagnan principle of a currency union: 'one for all and all for one'," said Barwell.
$1 = 0.8627 euros Writing by Paul Carrel; Additional reporting by Noah Barkin in Davos; Editing by David Stamp and Pravin Char