WASHINGTON — Housing and Urban Development Secretary Alphonso Jackson announced his resignation yesterday, leaving the Bush administration without a top housing official in the midst of a vast mortgage crisis that has shaken the global economy.

Mr. Jackson, a longtime friend of President Bush since they lived in the same Dallas neighborhood, departed after the White House concluded he had too many controversies swirling around him to be an effective Cabinet member, several HUD officials said privately. Mr. Jackson has been the target of accusations of favoritism involving HUD contractors for two years, and is under investigation by the FBI and Justice Department for allegations that he steered business to friends.

Several Democratic lawmakers demanded Mr. Jackson’s resignation last month after he refused to answer questions about the accusations, including a lawsuit filed by the Philadelphia Housing Authority against HUD that alleged that Mr. Jackson and his aides used the department to punish the authority for refusing to transfer valuable property to one of Mr. Jackson’s friends.

“There are times when one must attend more diligently to personal and family matters,” Mr. Jackson said in a prepared statement that he read at a news conference yesterday morning. “Now is such a time for me.”

Mr. Jackson said his resignation will be effective April 18. He took no questions and made no mention of the criminal probes or other allegations swirling around him. Mr. Bush, who departed yesterday on a trip to Eastern Europe for his final North Atlantic Treaty Organization summit, issued a written statement saying said he accepted the resignation “with regret.”

HUD is a $35 billion agency that funds public housing and rental assistance for low-income families. It also runs the Federal Housing Administration, which helps struggling and first-time buyers to buy and refinance their homes with a low down payment.