The release of the U.S. Producer Price Index (search) for May has been delayed indefinitely by renewed calculation problems, but figures already out are accurate, the Bureau of Labor Statistics (search) said Wednesday.

"We don't know definitively what our difficulties are. We don't know exactly what is causing the difficulties," Irwin Gerduk, assistant commissioner for BLS' PPI section, told Reuters. "The (past) numbers are right."

This is the latest in a string of calculation woes that have forced several delays, starting with January's index.

BLS spokesman Gary Steinberg said staff are working tifficulties with the index, which measures inflation and is also watched by economists because it can offer early clues on profits and, by extension, hiring.

The PPI was scheduled for release on Thursday at 3 p.m. EDT , itself a change from the original release date of Friday at 8:30 a.m., when government offices will be closed for a day of mourning for the late President Ronald Reagan (search).

Steinberg said the report would not be issued before June 15 and the department will give a day's notice ahead of any release of the index.

The PPI reports for January, February and March were delayed because government statisticians had problems switching the PPI to a new classification system. Aging computers took part of the blame.

Delays in the release of official U.S. economic data are very rare. The renewed problem has stirred market speculation that the BLS may have never sorted out the original trouble and that already released data may not be accurate.

"It does raise questions about whether or not there will be revisions when they do it, and there are the normal conspiracy theorists running around claiming it's going to mean bad news for inflation," said Lehman Brothers economist Drew Matus.

But Gerduk said the January-through-April data released since the first delay are accurate.

"We went through a very involved set of reviews and double checks to confirm that the data were accurate prior to publication. And we certainly didn't rush to release any of those months of data until we were convinced of the quality of the data was our usual level," Gerduk said.

The PPI measures prices paid to farms, factories and refineries and is used by economists and market-watchers to measure inflation pressures in the economy. Rising inflation is one of the reasons the Federal Reserve is expected to raise interest rates at the end of the month.

Matus said the PPI was not as important as the Consumer Price Index, which so far has shown little evidence that producer price pressures were feeding into consumer inflation.

The PPI soared 0.7 percent in April, the largest increase since a 1.3 percent spike in March 2003, with much of the increase from rising energy prices.

Gerduk said it was too soon to say whether the new problem was linked to when the BLS tried to switch the PPI to the North American Industry Classification System from the Standard Industrial Classification earlier this year, but economists said a separate problem seemed unlikely.

"If the first four were OK, then what changed between then and now? We know why the first was delayed, so I think it's all got to be related," Matus said.

"Spending a little extra money on the statistics agencies -- as (Federal Reserve Chairman Alan) Greenspan has argued for for years -- would probably be money well spent," he added.