(Repeats Thursday item)
* Investors see only one optimal QE option: ECB buys govt bonds
* National central bank purchases evoke euro zone fragmentation
* AAA-only bond purchases limit size and impact of QE
By Marius Zaharia
LONDON, Jan 8 (Reuters) - The ECB is considering three main options for pumping money into the struggling euro zone economy but two of them could hurt confidence in the bloc's most indebted states, defeating the object of the exercise.
With euro zone consumer prices falling in December, for financial markets it's no longer a question of whether the European Central Bank will act to boost economic growth and ward off a deflationary spiral, but when.
President Mario Draghi may announce an ECB programme of buying government bonds, using newly printed money intended to flood the wider economy, as soon as the Governing Council's next policy meeting on Jan. 22.
The main scenario for markets is that the ECB will join its U.S., Japanese and British peers in launching quantitative easing (QE) by buying government bonds in amounts proportionate to each euro zone state's shareholding in the bank.
But objections from the Germany's Bundesbank to the ECB taking on sovereign credit risk have raised two compromise solutions, as suggested by recent comments from ECB chief economist Peter Praet.
Option two is that national central banks buy the debt of their own governments, so the risk remains with the country in question. The third is the ECB buys only triple-A rated bonds, hoping that investors would turn to the lower-rated debt of weaker euro zone government which, while riskier, offers a better return.
However, economists believe the second and third options could backfire. If the ECB were unwilling to take on the risk of holding Greek, Italian, Spanish or Portuguese debt, private investors might ask themselves why they should do it.
The main scenario preferred by investors appears most in keeping with the solidarity principles of European monetary union. If the ECB had to take losses on the bonds of a member state which could not repay its debt, the central bank would have to be recapitalised by all 19 euro zone governments.
Private investors would also suffer losses in any debt write-off but at least the pain would be shared. By contrast, options two and three would not spread risk across the union; investors could therefore seek a premium on lower-rated bonds, pushing up yields for the governments of the very countries that need lower borrowing costs most.
"It seems like a step away from the whole notion of monetary union," said Luke Bartholomew, investment manager at Aberdeen Asset Management, a firm with over 400 billion euros of assets.