SAN JUAN/NEW YORK, May 1(Reuters) - Puerto Rico's governor on Sunday said he declared a moratorium on a $422 million debt payment due Monday by the island's Government Development Bank, the most significant default yet for the U.S. territory facing a massive economic crisis.

Governor Alejandro Garcia Padilla said in a televised speech that he signed the moratorium on Saturday in what he characterized as a "painful decision" based on inaction from the U.S. Congress, which continues to debate a legislative fix for Puerto Rico's $70 billion debt load.

(Reporting By Nick Brown and a contributor in San Juan, Daniel Bases in New York; Editing by Chizu Nomiyama and Diane Craft)