Moody's cut Greece's debt to A2 from A1 on Tuesday over soaring deficits, becoming the third major rating agency to downgrade the highly-indebted country's rating this month.

Moody's kept Greece on a negative outlook. Its rating is still two notches above that of Fitch and S&P, which earlier this month cut their rating on the indebted country to BBB+, the euro area's lowest level.

The one-notch downgrade was less than many investors had expected. The spread between 10-year Greek and German benchmark Bunds tightened over the news, and Greek shares opened slightly higher, although the euro trimmed gains.

"Greece's repositioned rating of A2 balances the Greek government's very limited short-term liquidity risks on the one hand, and its medium- to long-term solvency risks on the other," said Sarah Carlson, Moody's lead sovereign analyst for Greece.

"Moody's notes that the country's longer-term risks have only partly been offset by the government's announced policy response," Moody's Investors Service said in a statement.

The euro trimmed gains against the dollar in response, dipping to around $1.4302 from around $1.4323 before the announcement. It was last at $1.4309, up 0.2 percent on the day.

Moody's A2 rating does not threaten Greece's access to European Central Bank funds at the end of next year when the ECB plans to tighten its collateral rules.

But should Moody's downgrade Greece by a further two notches into 'B' territory, as Fitch and S&P have already done, come the end of next year, banks would no longer be able to exchange Greek government debt for cash in ECB refinancing operations.

"Moody's believes that Greece is extremely unlikely to face short-term liquidity/refinancing problems unless the European Central Bank decides to take the unusual step of making the sovereign debt of a member state ineligible as collateral for bank repurchase operations — a risk that we consider very remote," says Arnaud Mares, Senior Vice President in Moody's Sovereign Risk Group.

"Moody's also does not believe that the Greek government's difficulties represent a vital test for the future of the euro zone, but rather a repricing of relative risks that had been concealed by years of abundant global liquidity and somewhat above-potential growth," the statement said.

The Athens bourse's benchmark stock index, which has shed 11.9 percent in the last 30 days, was 2.42 percent higher in early trading.

"It was a relatively positive surprise, given the rumors circulating in the market for a 2-notch downgrade. Markets may bounce on the news," said economist Platon Monokroussos at EFG Eurobank.

Greece is set to become the euro zone's most indebted nation in terms of GDP next year, with public debt seen at 121 percent of GDP. The budget deficit widened to 12.7 percent of GDP in 2009. The government plans to cut it to 8.7 percent next year.