The Environmental Protection Agency said yesterday that the cities of Philadelphia and New York may continue until 1981 to dump sewage sludge in sites not far from the Maryland-Delaware coast.

Maryland officials and environmentalists have long been infuriated over the disposal sites, especially one near Cape May, N. J., just 35 miles from the Maryland-Delaware coast. Last fall, Maryland authorities appealed to the ERA to move the dumping operations to a new site 106 miles out in the ocean.

The state's officials protested that the dumping pollutes Maryland's shellfish industry, threatens resort beaches on the Atlantic Coast and poses other public health dangers.

EPA Assistant Administrator Thomas G. Jorling said yesterday he would not require the relocation because it might stall implementation of a law phasing out all ocean sludge dumping by 1981.

Such relocation could also add unnecessary costs to ocean disposal operations and make sludge monitoring more difficult, he said. The cost issue is significant, according to the EPA, because higher transportation expenses might curtail the cities' efforts to pay for the development of alternative means of disposal.

Although there is evidence that "some" problems result from dumping at the present sites, they are not enough to warrant a change of locations, Jorling said.

Maryland officials could not be reached for comment yesterday.

New York and neighboring communities now dump about 4 million tons annually at a site 12 miles out in the New New York Bight, while Philadephia's site off the Maryland-Delaware coast collects 750,000 tons annually.

The U. S. Food and Drug Administration already has forbidden the taking of any shellfish from an area adjacent to the Philadephia dumpsite.