Financial terms were not disclosed.

Founded in 2015, B2C2 is one of the best-known cryptocurrency trading companies. It helps brokerages, exchanges and fund managers make large trades in virtual coins.

SBI has been allowing its clients to trade through B2C2's platform since July, when it acquired a minority stake in the crypto firm.

The companies hope the acquisition will create the counterparty of choice for mainstream financial firms looking to invest in cryptocurrencies, they said.

"Their (B2C2's) vision, expertise and offering complement SBI's, and we look forward to working in partnership as we expand our footprint across the global markets," said Yoshitaka Kitao, president and CEO of SBI Holdings.

The deal, which marks the latest move by a large financial institution in crypto, could help make the nascent asset class more mainstream.

It comes as bitcoin continues to soar to record highs against the dollar, backed by growing demand from investors who see the virtual currency as a hedge against inflation and a safe-haven asset.

While cryptocurrencies have been around for more than a decade they have been attracting established financial companies over the last few years.

S&P Dow Jones Indices announced earlier this month that it would launch cryptocurrency indexes in 2021, while Fidelity Investments and Japan's Nomura Holdings Inc have started safeguarding virtual coins for institutional investors.

Such services are helping make the asset class more accessible for institutions.

"A lot of people have been dismissive for a long time," Max Boonen, B2C2's founder, said in an interview. "Bitcoin's price at an all-time high has put concerns to rest."

As part of the acquisition B2C2's team in Japan will move into the SBI's offices, Boonen said. He expects B2C2's team to grow to 70 people over the coming months from the current 50.

(Reporting by Anna Irrera in London; Editing by Matthew Lewis)

By Anna Irrera