The trustee liquidating MF Global Holdings Ltd's broker-dealer unit said on Monday the apparent shortfall of customer funds may be around $1.2 billion (766.2 million pounds), roughly double initial estimates from regulators.

The trustee believes that even if he recovers everything that is at US depositories, the apparent shortfall in what MF Global management should have segregated at US depositories may be as much as $1.2 billion or more, trustee James Giddens said in a statement. He added that the amount could change.

The Commodity Futures Trading Commission had previously estimated a shortfall of around $600 million, though Giddens had stressed he could not quantify the amount.

The CFTC and other regulators have been investigating whether MF Global may have improperly co-mingled customer money with its own funds.

Customers have been clamouring for answers -- and for access to their accounts -- since MF Global's customer accounts were frozen when the company went bankrupt on October 31.

MF Global was run by former Goldman Sachs & Co chief and New Jersey governor Jon Corzine before its Chapter 11 filing. The bankruptcy came after the New York-based company revealed that it made a $6.3 billion bet on European sovereign debt. Corzine resigned on November 4.

Giddens has transferred a portion of most accounts to other commodities brokers to give customers access to some of their funds. By early December, he said in the statement, he hopes to have transferred 60 percent of the total value of segregated customer accounts to other brokers.

He said the transfer would require $1.3 billion to $1.6 billion to implement, exhausting nearly all of the assets under his control.

Customers have criticized Giddens' efforts, calling for quicker access to a higher percentage of their accounts. The trustee has argued that a piecemeal approach is necessary until he can peg the exact size of the customer shortfall.

The trustee's case is In re MF Global Inc, U.S. Bankruptcy Court, Southern District of New York, No. 11-2790.

The MF Global bankruptcy is In Re MF Global Holdings Ltd, in the same court, No. 11-15059.

(Reporting by Nick Brown; Additional reporting by Jonathan Stempel; Editing by Tim Dobbyn)