European Central Bank head Mario Draghi has called off his trip to an annual Jackson Hole conference of central bankers this week due to a heavy workload as the bank works on its eagerly awaited plan to lower borrowing costs for struggling governments.

The ECB holds a key meeting Sept. 6 where plans to intervene in bond markets will be discussed. A spokesperson for the bank said Tuesday that Draghi decided not to go to the Wyoming meeting at the end of this week "because of the heavy workload foreseen in the next few days."

Draghi announced Aug. 2 that the ECB might help indebted governments lower their borrowing costs by buying their bonds — but only if the countries first ask for help from the region's bailout fund and agree to take steps to reduce their deficits and debt levels. He left key details blank and said committees of top bank officials would be working on them.

Several countries among the 17-strong group that uses the euro — including Spain and Italy — are struggling to borrow money at an affordable cost — because bond investors fear they may default. Yet the governments must constantly sell new bonds to pay off old ones that are coming due. High borrowing costs were what pushed indebted countries Greece, Ireland and Portugal to need bailout loans from the other eurozone countries.

Key questions remain about how the ECB purchases would work, such as how big they might be. An earlier round of purchases piled up over €210 billion ($263.5 billion) in bonds, but failed to decisively lower borrowing costs in part because ECB officials insisted the program was limited in size and duration.

Draghi's recent announcement said the next round of bond purchases would be "of a size adequate to reach its objective," without specifying how big that would be.

The ECB would first start buying bonds along with the eurozone's bailout fund, but only the ECB has the ability — in theory — to create new money to pay for the bond purchases, giving it potentially much vaster financial firepower.

It's also not clear whether the ECB will set a target interest rate — or yield — that a country's bonds would not be allowed to exceed when it launches purchase program. Buying bonds drives their prices up and their interest yields down, since price and yield move in opposite directions. Setting a specific target could be risky, as the ECB would have to defend it in markets or lose credibility. Spanish 10-year bonds were yielding a painfully high 6.4 percent Tuesday.

Spanish officials have pressed to know more details about the ECB's intentions before they take the politically painful step of asking for a bailout. The bond purchases remain controversial in some quarters, with the head of Germany's Bundesbank national central bank, Jens Weidmann opposed. Weidmann has said goverments might find central bank help "addictive as a drug" and might lose the will to make painful decisions to reduce spending. He also argues it is too close to using the central bank's monetary powers to finance governments — something the EU treaty forbids it to do directly.

The ECB could also face further wrangling over a proposal due Sept. 11 from the European Union's executive body, the Commission, to create a centralized EU banking regulator under the aegis of the ECB. The plan, launched at a summit of European leaders in June, is designed to strengthen Europe's banking industry against further financial shocks.

A top ECB official, Joerg Asmussen, said in a speech Monday that the ECB would refuse to take over the job unless it is given the power to close down insolvent banks — a power that could bring it into conflict with national governments and regulators who have been protective of their home banks.