London » The United States and Britain are more likely than Germany and France to witness an embarrassing downgrade of their top debt rating, agency Moody's Investors Service said Monday.

In a quarterly report assessing the prospects of the triple A-rated countries, including Spain and the "less fiscally challenged" Denmark, Finland, Norway and Sweden, Moody's warned the economic recovery remained fragile in many advanced economies.

Governments and central banks are looking at when and how to unwind their massive stimulus measures, which include historically low interest rates, liquidity provisions, industry incentives and increased spending. Although some experts warn that exiting these policies too early risks creating a new economic downturn, they are also straining government finances.

"This exposes governments to substantial execution risk in the implementation of their exit strategies, which could yet make their credit more vulnerable," said Arnaud Mares, senior vice president in Moody's sovereign risk group and the main author of the report.

For now though, Moody's said the triple A governments don't face an immediate threat to their top ratings as the servicing of the debt remains manageable -- the top credit rating reduces the interest payments countries have to pay on their debt when going to the bond markets to raise capital.

However, debt affordability is "most stretched" in Britain and the U.S., Moody's said.

In light of the muted recovery from recession in many countries, Moody's said government action on spending and taxes is the main way of "repairing the damage" that the global crisis inflicted on government finances.

The debate about when to start cutting spending is likely to be at the heart of the general election campaign in Britain, which is expected to formally kick off in the next few weeks -- most commentators think that Prime Minister Gordon Brown will call an election for May 6 early next month.

Economists warn that Britain is on course to borrow the equivalent of 12.8 percent of gross domestic product in 2009/10 -- exceeding the 12.7 percent forecast in crisis-hit Greece and far above the average 6 percent for Europe.

In the U.S., the budget deficit this year is projected to be just under 10 percent of the economy, meaning that the Treasury has to sell more and more bills to fund the shortfall.