The Internal Revenue Service's Philadelphia service center said last week that it drastically has cut a backlog of unprocessed returns, but that 18,597 persons still are awaiting refunds due since June 1.

A new computer system has been blamed for delays and backlogs in the Philadelphia center, which serves taxpayers in Pennsylvania, Maryland, Delaware and the District of Columbia, and for problems at nine other processing centers nationwide.

Computer glitches in Philadelphia were especially serious, and about 160,000 persons had to refile their 1984 returns after the IRS lost them, said James Davie, an IRS spokesman.

Sen. John Heinz (R-Pa.) has been critical of the glitches, and an aide to the senator said that the IRS is "putting a good deal of effort into trying to get the problem fixed." But the aide, who asked not to be identified, added: "We're still getting letters here -- much more than would be the norm -- from people still waiting to get refunds."

The IRS has issued refunds to all but 15,913 of those who resubmitted their returns to the Philadelphia service center, Davie said.

Interest penalties paid by the Philadelphia IRS office on 506,000 late refunds, including the returns, totaled $14 million, said Steven Pyrek, an IRS spokesman here.

Nationwide, the agency has paid $41 million in interest on refunds issued after June 1 because of the computer problems.

Federal law requires the IRS to mail refund checks by June 1 for returns filed by April 15. After June 1, the agency must pay interest of 11 percent.

The Philadelphia service center handled 8.2 million returns this year.