UBS AG has joined Bank of America in closing down some options for wealthy clients to access billionaire investor John Paulson's Advantage fund, sources said on Thursday, after the portfolio suffered double-digit losses in 2014.

UBS shut down its $50 million feeder fund at the end of July, roughly the same time that Bank of America told its financial advisers they would have to liquidate clients' investments in the fund, once one of Paulson's biggest offerings.

The banks cited concerns about costs, plus bets on illiquid securities as reasons for their moves, according to the sources. UBS clients will still be allowed to invest in Advantage but through a different share class, and it is unclear how many will elect to do so.

Financial advisers at both banks, who had put money into Advantage when Paulson was making lucrative bets on housing and gold several years ago, said they have become concerned about the fund's volatile returns since 2011.

It fell 19 percent in 2014 when it was caught wrong-footed on a failed pharmaceuticals merger, reversing most of a 34 percent gain in 2013, and added to heavy losses the previous two years. It was up 2.2 percent in the first half of 2015.

"Why would you want to keep your money in a fund that isn't performing well?" asked one UBS financial adviser. "I wasn't surprised when the bank told us about six weeks ago that they would kill the feeder fund on July 31."

One person who invested through UBS said he would be getting back only about $140,000 of the $250,000 he'd put into the UBS feeder fund five years ago.

The banks' decision echo similar moves by Morgan Stanley and Citigroup, which pulled out of Advantage in 2012 as the fund's performance began to weaken. JP Morgan never offered its wealthy clients access to Advantage.

UBS launched the Advantage feeder fund in 2009, requiring investors to commit a minimum of $250,000. The fund had about $18 billion under management in 2011, a figure that has dwindled to roughly $2.8 billion since then due to redemptions and lackluster performance.

To be sure, Paulson's $20 billion firm remains one of the hedge fund industry's biggest and the 59-year old investor ranks among its most closely followed.