Ma Bell revealed yesterday that because of an "undetected electronic file transfer error" which omitted information from its March Securities and Exchange Commission filing, the company will be forced to delay by a month the planned exchange of $10 billion of AT&T; stock for shares in AT&T; Wireless.

The exchange offer is now due to take place in April. But AT&T; stressed the change will not delay the timing of the Wireless split-off this summer.

The computer snafu is yet another setback for the giant, which plans to split into three new companies and create a tracking stock for the consumer long-distance unit by next year.

The company has reduced profit forecasts four times since May. And it has already pushed back the initial public offering for its broadband unit to September.

To top it all off, AT&T; has lost more than half its value in the last year.

Analysts say the filing error is just one new indication of the risks involved in executing the complicated restructuring.

In addition to the filing glitch, AT&T; said it would retain $3 billion of AT&T; Wireless stock after the spinoff to sell as a debt-reduction move. Ma Bell previously said it would distribute all of its remaining stake in AT&T; Wireless to AT&T; shareholders.

The about-face clearly surprised some investors. AT&T; fell more than 3 percent to $21.76.