By Adrian Croft

LONDON (Reuters) - Prime Minister Gordon Brown and French President Nicolas Sarkozy will urge banks this week to make "full and immediate disclosure" of write-offs due to the global credit crisis, British officials said on Monday.

The two leaders are increasingly concerned that confidence in financial markets is being hit by uncertainty over the scale of bad debts on banks' books, which some estimates put as high as $600 billion (303.2 billion pounds), Brown's office said.

Sarkozy is due to hold talks with Brown on Thursday during his two-day visit to Britain as guest of Queen Elizabeth.

The two men will "call for greater transparency in financial markets and, as a first step, full and immediate disclosure of the scale of write-offs by banks," Brown's office said in a statement.

The bank crisis, especially the collapse of U.S. bank Bear Stearns, had shown "the scale of the problem and the effect on market stability of difficult-to-value assets and of undisclosed losses becoming known in a piecemeal fashion," it said.

Mortgage-backed securities have plunged in value amid a credit squeeze sparked by low quality mortgages in the United States, leading to a vicious circle of forced sales, falling prices and weakening balance sheets for banks.

Banks have written down over $125 billion of assets since November, hammering their shares.

So far, action by central banks and governments has failed to halt the market turmoil.