LONDON—A planned pan-European payments network backed by 22 banks to compete with U.S. duo Mastercard and Visa appealed on Tuesday for public money, saying its private backers were not prepared to stump up all the cash needed. The European Payments Initiative (EPI) was launched last year and is seeking to get more banks and other players in payments on board by December to finalize plans to start rolling out its instant and card payments network. “Public funding would be nice,” EPI Chief Executive Martina Weimert told an event held by the European Payments Institutions Federation. “Let’s not hide it—it’s going to be a massive investment. It’s expensive.” Retailers say they are not ready to pay for it, while banks and other EPI shareholders “can carry only so much,” Weimert said, without giving details on the investment needed. Deutsche Bank, UniCredit, BNP Paribas, ING, Societe Generale, and Sabadell are among the …