(Bloomberg) -- BlackRock Inc., the world’s largest issuer of exchange-traded funds, will move most of its U.S. ETF assets from State Street Corp. to reduce its reliance on the bank’s custody services. 

BlackRock, with $2.2 trillion of U.S. ETF holdings at the end of the third quarter, will shift 40% to Citigroup Inc., 30% to JPMorgan Chase & Co. and 15% to Bank of New York Mellon Corp., BlackRock said Tuesday in a statement. The rest will stay with Boston-based State Street, one of BlackRock’s biggest rivals in the ETF business.

State Street, the third-largest issuer in the U.S. after BlackRock and Vanguard Group, oversees the biggest ETF -- the SPDR S&P 500 ETF Trust, with about $420 billion of assets.

“These changes reinforce and diversify our operational foundation so that we can deliver more ETF exposures at greater scale and with the high standards that our clients expect,” Salim Ramji, BlackRock’s global head of iShares and index investing, said in the statement.

BlackRock, which spent two years considering the change, is still weighing a similar shift for its Ireland-based ETFs. 

The U.S. transition will start in the second half of next year and take 18 months to complete, BlackRock said. It follows another large shift in 2017, when the New York-based firm yanked $1 trillion from State Street’s supervision and moved it to JPMorgan to cut costs.

“State Street will continue to play a critical role going forward as a long-term partner to iShares and the firm,” bank spokesman Ed Patterson said in an emailed statement. “Beyond ETFs, State Street continues to be a critical service provider to a diverse set of BlackRock funds.”

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