Moody's Investors Service has downgraded Greece's government bond rating to Caa3 from Caa2 on Wednesday. The agency placed the rating on review for further downgrade.

Moody's said that it cut the rating because the probability of support continuing to be provided over the medium-term has fallen since April.

"Moody's believes that without ongoing support from official creditors, Greece will default on its privately-held debt. Events of recent months have illustrated the distance between what Greece's official creditors will demand as a condition of continued support over the coming years, and what Greece's institutions are able to do to meet those demands with further meaningful economic and fiscal reforms. This creates significant difficulties for the achievement of a long-lasting support agreement," the agency noted.

Moody's added that a "no" vote in the referendum would increase risk of the exit from the Eurozone which could lead to significant losses on private sector creditors.