WASHINGTON — President Bush early this morning signed a massive housing bill intended to provide mortgage relief for 400,000 struggling homeowners and stabilize financial markets.

Bush signed the bill without any fanfare or signing ceremony, affixing his signature to the measure he once threatened to veto, in the Oval Office in the early morning hours. He was surrounded by top administration officials, including Treasury Secretary Henry Paulson and Housing Secretary Steve Preston.

White House spokesman Tony Fratto said the Federal Housing Administration would begin right away to implement new policies "intended to keep more deserving American families in their homes."

The measure, regarded as the most significant housing legislation in decades, lets homeowners who cannot afford their payments refinance into more affordable government-backed loans rather than losing their homes.

It offers a temporary financial lifeline to troubled mortgage companies Fannie Mae and Freddie Mac and tightens controls over the two government-sponsored businesses.

The House passed the bill a week ago; the Senate voted Saturday to send it to the president.