Moody's cut Greece's credit rating by three notches and kept it on review for further downgrades on Monday, citing significant risks to its fiscal restructuring program and the chance of a voluntary debt restructuring.

Moody's now has the lowest rating for Greece of all the major credit agencies and is the first to classify Greek government debt as 'highly speculative'.

The fiscal consolidation measures and structural reforms that are needed to stabilize the country's debt metrics remain very ambitious and are subject to significant implementation risks, Moody's said in a statement.

It added that it saw risks that conditions attached to continuing financial aid after 2013 will reflect solvency criteria that the country may not satisfy, and result in a restructuring of existing debt.

The spread on 10-year Greek debt against benchmark Bunds widened by 8 basis points to 9.13 percent following the Moody's downgrade, while the euro fell about 30 pips against the dollar.

(Reporting by Angeliki Koutantou; Writing by Toby Chopra; editing by John Stonestreet)