Yesterday the U.S. Treasury Secretary, Henry Paulson, nationalized the underwriters of half of America’s vast mortgage industry, now in precipitate decline.

Secretary Henry Paulson at news conference yesterday

Two government-sponsored enterprises (GSEs), Fannie Mae and Freddie Mac, have had their books underwritten in the short term by the Government.

Fannie and Freddie are curious institutions. They don’t lend money but underwrite half of U.S. mortgage lending. This amounts to a staggering $5.2 trillion (£2,940 billion) of debt. The two companies are a kind of buffer zone between mortgagees and the real world of finance.

Since the Great Depression, the Government has tacitly made it known that it will support Fannie and Freddie through any adversity. Now that wink has become explicit — until the end of 2009.

So the managers of these enterprises are out, and the shareholders are sent to the dogs, losing 79.9 percent of their holdings to the Treasury. The bondholders — most central banks and commercial banks around the world — are safe, by Government decree. The alternative would have been a liquidation of dollar holdings on an unimaginable scale.

Predictably, bank shares have risen sharply around the world, while the dollar has lost some of its recent glitter in the markets, reflecting the new self-imposed straitjacket binding the Government’s hands for the foreseeable future.

Henry Paulson explained the thinking behind the move. “Fannie Mae and Freddie Mac securities are held by central banks and investors around the world. Investors have purchased securities of these enterprises in part because the ambiguities in their congressional charters created a perception of government backing. Because the U.S. Government created these ambiguities, we have a responsibility to both avert and ultimately address the systemic risk now posed by the scale and breadth of the holdings of GSE debt and mortgage-backed securities.”

He has also committed the Treasury to pumping up to $100 billion into each of the GSEs in the event that their capital ratios fall short.

Fannie and Freddie will now be able “moderately” to increase their lending.