First Data Corp., whose investors are led by KKR & Co., is preparing to seek at least $2.5 billion in what would be the biggest U.S. initial public offering this year, according to people with knowledge of the matter.

The stock could start trading by the end of this month, according to the people, one of whom described it as an acceleration of earlier plans. The price and timing are subject to change depending on stock-market volatility and how investors react to a pending roadshow, the people said. First Data filed in July to sell stock, without specifying a fundraising target or timing.

First Data is KKR’s biggest equity bet ever: Eight years ago the buyout firm took the payment company private for $29.8 billion, including debt and fees. The planned stock sale would leap past other U.S. IPOs this year, such as Tallgrass Energy GP LP’s $1.38 billion offering and Fitbit Inc.’s $841 million, according to data compiled by Bloomberg.

Spokesmen for First Data and KKR declined to comment.

First Data runs a debit-card network, processes bank-card transactions and provides data analytics and other services to merchants. Proceeds from the IPO will be used to repay debt.

The company selected Citigroup Inc., Morgan Stanley and Bank of America Corp. to manage the offering, according to a regulatory filing last month. KKR will help underwrite the deal. Barclays Plc and Credit Suisse Group AG also are among the 15 underwriters managing the offering.

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