By Balazs Koranyi and Francesco Canepa

FRANKFURT (Reuters) - The European Central Bank is all but certain to keep rates firmly on hold on Thursday but will have to address an ever growing list of obstacles that threaten once again to derail its efforts to revive growth and inflation.

Italy's bank troubles, Britain's decision to leave the European Union, and a scarcity of bonds to buy in its asset purchase program may all require some action, dashing ECB chief Mario Draghi's hopes that the bank was done after years of extraordinary stimulus measures.

Not keen on hasty moves, Draghi is likely to maneuver through with verbal action, highlighting the increased risks and opening the door to changes as soon as September, when the bank releases fresh economic forecasts.

The trick will be to sound dovish enough to signal readiness to act but without committing.

The bank will announce its rate decision at 7.45 a.m. ET and Draghi will hold a news conference at 8.30 a.m. ET.

The ECB is buying 1.74 trillion euros ($1.91 trillion) worth of assets to cut borrowing costs, induce spending, lift growth and ultimately raise inflation, which has been stuck either side of zero for the past two years.

Brexit may be the biggest single problem, threatening to thwart a modest investment and consumption-led recovery. But the ECB simply does not have enough information to work with.

Early data, such as Germany's ZEW sentiment indicator and euro zone consumer confidence figures, suggest a significant drop in confidence. But such surveys are prone to sharp swings and the ECB would need a larger body of evidence to act.

Indeed, while analysts polled by Reuters cut their 2017 euro zone growth forecasts to 1.3 percent from 1.6 percent, they left their inflation projection unchanged at 1.3 percent, a mixed reading for the ECB, which targets inflation at just below 2 percent.

Draghi is expected to argue that Brexit is a political problem, requiring governments, not the central bank to act, a call that is likely to fall on deaf ears, much like his repeated pleas for structural reforms that could lift potential growth.

"Mr Draghi's calls for a loosening of the purse strings will go unheard, at least for now," BNP Paribas economist Luigi Speranza said. "As things stand, then, the burden of responding to the Brexit shock will remain with the ECB, which is all too aware that it has fewer and fewer tools with which to respond."

CONSTRAINTS

With interest rates deep in negative territory and asset buys running at 80 billion euros per month, the ECB is indeed running short on tools, raising the threshold for any further move if the bank is to preserve some firepower for any future shock.

But some policy changes are still likely in the coming months because the ECB is running out of qualified assets to buy, particularly Germany government debt, as yields fall below its -0.4 deposit rate, a self-imposed limit for its buys.

The dilemma will be whether to tweak the scheme, making just technical changes, or enact a broader but more controversial shift that could fundamentally alter the nature of the ECB's quantitative easing.

"Technical problems can be solved but they underline the limits of what can be achieved by using solely monetary policy – something Draghi is very much aware of and one of the reasons why he constantly calls for a more growth-friendly fiscal policy," financial services group Nordea said.

Analysts polled by Reuters see mostly technical changes for now, including purchases of bonds yielding less than the deposit rate and buying a bigger portion of bonds without collective action clauses.

More fundamental changes may still come later, especially as the vast majority of analysts expect the asset buys to be extended beyond their current scheduled end next March and technical tweaks may not be enough to accommodate a major extension.

Italian banks, weighed down by about a 360 billion euro ($400 billion) bad debt and falling share prices, are also likely to be on Draghi's agenda.

The Italian government is in talks with the EU to allow state aid to the troubled lenders but wants to shield household investors, a contentious proposal that would test the EU's bail-in rules.

Though the ECB has no direct say in the matter, it still has a vested interest as it is the sector's main supervisor and banks are the main transmitters of its monetary policy.

The ECB has also argued that European rules leave room for state aid, referring to provisions which allow for shielding private investors from losses to preserve financial stability.

(Editing by Jeremy Gaunt)