British billionaire investor Joseph Lewis has acquired a 7 percent stake in brokerage Bear Stearns at a time when the troubled firm has suffered significant losses due to the subprime mortgage crisis.

Purchases through Bahamas-based companies controlled by Lewis in the past several weeks amount to more than 8 million shares worth about $860 million, according to a Bear Stearns filing with the Securities and Exchange Commission.

Lewis, 70, is a currency trader who at one point owned almost 30 percent of auction house Christie's. The purchases make him the company's largest shareholder.

Earlier this summer, Bear Stearns reported that two of its hedge funds had suffered losses of more than $1 billion, driven by defaults on subprime mortgages. Shares of Bear Stearns have fallen more than 30 percent this year as a result of the crisis.

Shares received a boost from investors today, however, rising $2.13, or 2.02 percent, to reach $107.50 at the end of normal trading hours.