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July 31 (Reuters) - Puerto Rico will miss a payment on debt due Aug. 1, the governor's chief of staff said on Friday, an event that will be considered a default by investors as the commonwealth lurches towards what could be one of the largest U.S. municipal debt restructurings in history.

The missed payment will mark the first default by the commonwealth and shows the depth of the island's economic and cashflow problems. Puerto Rico Governor Alejandro Garcia Padilla shocked investors in June when he said the island's debt, totaling $72 billion, was unpayable and required restructuring.

According to a 2014 bond offering statement, Puerto Rico has never defaulted on the payment of principal or interest of debt.

"Tomorrow is Aug. 1 and we don't have the money," Victor Suarez, chief of staff for Puerto Rico's governor, told journalists in San Juan, referring to a $58 million payment due on Public Finance Corporation (PFC) bonds.

"The PFC payment will not be made this weekend," Suarez said. "It was not consigned."

The island faces a number of debt payments due Aug. 1 but had signaled in recent weeks that it may miss the PFC payment.

The non-payment would be the most notable since Detroit, which had about $8 billion of bonds, defaulted on $1.45 billion of insured pension bonds before it filed for bankruptcy in 2013.

"What could surprise investors is when they actually hear the word 'default,' and that a default occurred," said Lyle Fitterer, head of tax-exempt fixed income at Wells Capital Management, which holds mostly insured Puerto Rico debt.

"The immediate reaction might be a slight sell-off in the marketplace because I think people will start to anticipate, 'OK, what's the next series of debt they're going to default on?'"

PFC bonds have weaker protections than many other Puerto Rico bonds and the skipped payment had been signaled to investors over the past few weeks. Suarez said on Monday that the commonwealth did not have the current cash flow to pay the PFC bonds.

"I bought my (PFC) bonds with the anticipation of them defaulting," said Ben Eiler, managing partner at First Southern Securities in Puerto Rico, earlier this week. "They're going to restructure in some form or fashion, and I believe that restructure is going to be higher than that level."

However, Puerto Rico is making another debt payment due. The head of its Government Development Bank (GDB), President Melba Acosta, said in a statement released Friday that the bank would make a $169 million payment for the debt service on GDB debt.