Billionaire Joseph Lewis disclosed in a regulatory filing on Wednesday that he has increased his stake in hard-hit investment bank Bear Stearns Cos to 9.57 percent.

Lewis, who made his fortune trading currencies, has been building his stake in Bear Stearns over the past few months. In September, he paid $860.4 million for a 7 percent stake which he then increased to 8.01 percent earlier this month.

Lewis, a reclusive Englishman who splits his time between homes in Florida and the Bahamas, is Bear Stearns largest shareholder, according to Reuters Data.

The latest filing with the U.S. Securities and Exchange Commission showed that Lewis' funds bought shares from December 6 through December 21 at prices varying from $89.08 to $110.00.

Lewis and several investment vehicles he controls paid a total of $1.19 billion for the stake, the filing said. He paid an average of $101.07 per share in his latest foray.

Based on Bear's closing share price of $88.80 on Monday, Lewis' stake is worth about $985.68 million, leaving him down roughly $200 million.

Shares have fallen roughly 17 percent since Lewis first announced he had built a stake in the company in September.

Since the beginning of the year, Bear Stearns shares have plunged more than 45 percent as the slumping mortgage market hurt the bank's biggest businesses and prompted write-downs on mortgage-related assets. Bear also suffered a hit to its reputation as a savvy bond trader in July when two mortgage hedge funds it managed were forced into bankruptcy.

Lewis' stake exceeds that of Bear Stearns Chairman and Chief Executive James Cayne, who holds about 5.8 percent of the company's stock, including restricted stock and incentive-plan shares.

Lewis' buying of a larger stake in Bear is the latest significant change in the bank's shareholder base, after China's CITIC Securities Co (600030.SS: Quote, Profile, Research) and Bear forged a deal in October to swap stakes in each other, giving Bear greater access to China's burgeoning stock brokerage business and aiding CITIC's global expansion plans.

Bear said it would buy $1 billion of CITIC debt that would convert over time to a 2 percent stake in the Beijing-based firm, while CITIC would invest about $1 billion in Bear Stearns securities that would convert to about 6 percent of the New York-based investment bank.

Lewis' investment vehicle, Tavistock Group, holds interests in more than 170 companies in 15 countries, including English Premier League soccer club Tottenham Hotspur, golf course developments in Florida, Alcatraz Brewing Co and life sciences companies. He once owned about a third of auction house Christie's.

Forbes this year put Lewis at No. 369 on its world ranking of billionaires, with a net worth of $2.5 billion.

Bear Stearns stock was up 50 cents to $89.30 in afternoon trading on the New York Stock Exchange.