LONDON — Greece launched a tirade against U.S. credit ratings agencies Monday after Moody’s downgraded its debt grade further below junk status, warning the bailed-out euro country might have to default on its massive borrowings.

The agency slashed its rating by three notches to B1 from Ba1 and warned it may cut again if the government’s commitment to austerity wanes or international creditors become less willing to support it. Greece was saved from bankruptcy last May after accepting a $154 billion bailout from the EU and the International Monetary Fund.

The Greek government’s response was quick and critical. It said Moody’s downgrade was “completely unjustified” and “does not reflect an objective and balanced assessment” of Greece’s actual economic prospects.

It said credit rating agencies — rivals Standard & Poor’s and Fitch Ratings have also downgraded Greece heavily in past months — were trying to make up for failing to predict the 2008 global financial crisis.

In explaining its downgrade, Moody’s warned that the Greek government’s economic program may not yield the intended drop in debt and return to growth, and noted Greece’s considerable difficulties in raising revenue. It also highlighted the risk of tougher debt conditions when the EU-IMF bailout package ends in 2013.

“The risk of a post-2013 restructuring might lead the Greek authorities and investors to participate in a voluntary distressed exchange before that time,” Moody’s Investor Services said.