NEW YORK (Reuters) - Robinhood, the online brokerage used by many retail traders to pile in to heavily shorted stocks like GameStop Corp, has made an ambitious push into loaning out its clients’ shares to short sellers as it expands its business. The broker had $1.9 billion in shares loaned out as of Dec. 31, nearly three times the $674 million a year earlier, and it was permitted to lend out $4.6 billion worth of securities under margin agreements, around five times bigger than the prior year, according to an annual regulatory filing late on Monday. The size of the jump highlights Robinhood’s rapid growth over the past year as the number of retail investors has soared in the work-from-home environment during the pandemic and as retail brokers have largely eliminated trading fees, a model Robinhood helped pioneer. Menlo Park, California-based Robinhood is expected to go public this year with a valuation of more than $20 billion. Securities lending is common among brokerages, which can earn income by lending shares to hedge funds and others, who then sell the shares back into the market, betting the share prices will drop so they can buy them back at a lower price when it is time to return them, pocketing the difference. Shares that are in heavy demand from short sellers, like GameStop, which had 140% short interest in January, command the biggest premium from the lender. What makes Robinhood notable is that many of the stocks its users invest in are among the most sought-after by people who want to bet against them, said one senior financial executive involved with hedge funds. It was unclear how great a benefit the securities lending was to Robinhood’s revenue and income, which it does not disclose. Robinhood declined to comment. The brokerage does not allow short selling on its platform. In January, retail investors coordinated through trading forums on social media in an attempt to punish hedge funds by buying up shares of GameStop and other heavily shorted names, like AMC Entertainment, driving up their prices and forcing short sellers to close out positions at big losses. On Jan. 28, at the height of the retail trading mania, Robinhood, along with several other brokers, restricted the buying of GameStop and other so-called meme stocks due to a massive spike in collateral requirements needed to clear the trades, angering many of its customers. On Tuesday, U.S. President Joe Biden’s pick to lead the Securities and Exchange Commission promised a thorough review of issues raised by the GameStop saga, as well as other issues related to retail brokers and investors. The trading restrictions had already sparked congressional hearings, regulatory probes and placed greater scrutiny on short selling. In response, Vlad Tenev, Robinhood’s chief executive officer, called for shorter stock settlement times, which would reduce clearing collateral requirements. He also said the idea that more shares of a stock can be shorted than there are available to trade, as was the case with GameStop, is a “pathology” that could destabilize the financial markets. Robinhood positioned itself for growth in securities lending in October 2018 by launching its own clearing broker, which acts as a go-between with the clearinghouse that settles its trades, and allows it to hold its customers’ assets. The broker can then lend out securities its customers buy on margin. At present, less than 3% of Robinhood’s funded accounts are margin-enabled, Tenev recently told Congress.