BRUSSELS/ATHENS (Reuters) - Euro zone finance ministers approved a giant 30-billion-euro ($40 billion) emergency aid mechanism for debt-plagued Greece yesterday but stressed Athens had not requested the plan be activated yet.

Together with at least 10 billion euros expected from the International Monetary Fund in the first year, it could add up to the biggest multilateral financial rescue ever attempted.

"With today's decision, Europe sends a very clear message that no one, any longer, can play with our common currency, no one can play with our common fate," Greece's Prime Minister George Papandreou said in a statement.

In a rare weekend telephone conference, finance ministers of the 16 nations that share the single European currency backed a detailed plan for Greece to borrow from euro-zone governments and the IMF at significantly below market rates.

A Greek Finance Ministry official said it was logical to expect the package would amount to significantly more than 40 billion euros over three years. Earlier in the day, he had said it could hit 80 billion euros, but later corrected this.

If Greece obtains aid, the package could dwarf past IMF bailouts for Mexico and Argentina. The largest IMF commitment ever made to a country was the $47 billion arrangement for Mexico approved in April 2009 under a so-called flexible credit line; Mexico has not drawn from the credit line.

The firepower in the Greek package, even if held in reserve, may reassure investors and make them more willing to continue buying Greek bonds. But big uncertainties remain over the longer-term prospects for reducing Greece's debt mountain, which have dented confidence in the euro.

The Greek official said the government would decide within a few days whether to ask for the aid, depending on whether market interest rates subside.

European Economic and Monetary Affairs Commissioner Olli Rehn said the three-year euro zone loans would carry an interest rate of about five percent — well below current market rates of about 7.3 percent. That responds to Greece's appeal to be able to borrow at rates closer to its peers in the currency area.

Assistance for subsequent years would be decided later.

"If the mechanism had to be activated, it would not be a violation of the no-bailout clause (in the European Union treaty) since the loans are repayable and contain no element of subsidy," Jean-Claude Juncker, chairman of the Eurogroup of finance ministers, told a Brussels news conference.

A German government official welcomed the agreement, which he said should enable Greece to do its fiscal "homework" on deficit reduction without market distraction.

"It should contribute to a calming of the markets so that Greece can take care of its homework in peace and quiet."

Rehn said all euro zone countries would pay proportionately to their share in the ECB's capital, making Germany by far the biggest lender, followed by France and Italy.

Talks on coordination with the IMF will begin today, he said.