DENVER (AP) -- A computer hardware problem caused more than 800,000 credit and debit card transactions to be double- or triple-billed last week at Wal-Mart stores nationwide, according to officials at First Data Corp., which handled the electronic payments.

The excess Visa and MasterCard charges, which occurred Wednesday and were posted on Thursday, have been reversed, First Data spokeswoman Staci Busby said Sunday.

Busby said the problem showed up on reports the company generates for quality control purposes. She said it's unclear how many customers were affected, and that she had no other details about the hardware problem.

"Anyone who conducted a transaction with a Visa or MasterCard on March 31 should check their statements," Busby said.

Danetta Thompson, a spokeswoman for Bentonville, Ark.-based Wal-Mart said Sunday the retailer had posted signs about the problem and was informing customers. She said signs in the stores direct customers to First Data's toll-free number, 888-893-0626.

"I feel like they've got their arms around the problem," Thompson said. "Those charges have been reversed."

Busby said some affected customers may not see the reversed charges on their accounts until Tuesday. She said First Data has instituted additional measures as a result of the overbilling.

First Data processes payments for retailers, moving money from consumers' accounts to merchants' bank accounts. The Greenwood Village, Colo.-based company has 30,000 employees and operates in 195 countries and territories