The Internal Revenue Service, struggling to catch up with refunds delayed by computer foul-ups, was buried under a record crush of last-minute returns, the tax collection agency said yesterday.

Returns received by IRS service centers the week after the April 15 filing deadline showed an increase of 43.4 percent over the same period last year, an IRS spokesman said.

The additional number of taxpayers who chose to wait until the deadline this year was an unwelcome surprise for the IRS because its work was plagued by problems with its new computer.

The week ending April 26 saw 5.43 million returns show up at the agency's 10 service centers for processing, compared with 3.79 million last year.

The IRS service center in Philadelphia, where several particularly severe processing problems have been identified, is still 20 percent behind where it was last year. The center serves the District of Columbia and Maryland.

But the center is catching up. The week before, it was 31 percent behind the 1984 pace.

Refund checks for the year so far are running 8.7 percent behind last year, representing $2.6 billion being withheld from consumers.

The delayed refunds amounted to a lot more in January, February and March, however. The Commerce Department independently estimated that consumers had $6.8 billion less in those months because of the IRS snafu, possibly helping to slow the nation's economy.

So far this year, the IRS has certified, although not necessarily mailed, $34.9 billion in refunds, compared with $37.5 billion in the same period last year. The average refund for the year so far is $777.50, or 1.8 percent more than for the same period last year.

Because of its push to get payments out before June 1, when the agency becomes liable for interest at 13 percent on overdue refunds, the refunds processed each week are running far ahead of last year. The week ending April 25 saw the IRS certify 5.7 million refunds, 34.7 percent more than in the same week last year.

The agency still promises that all refund returns that can be processed immediately will be completed byt May 31.