By Anastasija Johnson

NEW YORK (Reuters) - Recent optimism that a global credit crisis was winding down took a severe blow on Monday when Standard & Poor's cut debt ratings of three U.S. brokers, a top U.S. bank ousted its chief executive and a British lender offered a bleak assessment of the UK housing market.

The day's first assault on investor confidence came when Bradford & Bingley, a specialty mortgage lender, reported a surprise loss and said Britain's housing market was deteriorating rapidly.

That was followed by Wachovia Corp, the No. 4 U.S. bank, ousting its chief executive and feeding speculation that more loan losses could be in the offing. Rival bank Washington Mutual, meanwhile, stripped its CEO of the additional role of chairman.

The final jolt came when Standard & Poor's, one of the three main credit ratings agencies, cut debt ratings on Lehman Brothers, Merrill Lynch and Morgan Stanley, citing a weakening profit outlook for the beleaguered sector.

The day's barrage served as a rough reminder that a crisis of confidence that has gripped global credit markets since mid-2007 may be far from over.

"It just reminds everyone how tenuous the situation is in the financial system now," said Carl Lantz, U.S. interest rate strategist at Credit Suisse in New York.

Investors and economists had begun to postulate that the near collapse of Bear Stearns in mid-March marked the low point of the cycle, which has its roots in the slump in the U.S. housing market.

Amid that optimism global stock markets had been grinding steadily higher since late March.