His departure is the latest in a series of resignations from the troubled interactive business of AOL Time Warner.

It comes just weeks before AOL chief executive Jon Miller was due to unveil a recovery plan for the loss-making online business.

The online division has been suffering from the slump in advertising revenue, low subscriber numbers and a probe into the company's accounting practises.

Short stay

Mr de Castro was appointed president of Interactive Services in April by the former AOL Time Warner chief operating officer Robert Pittman.

Maybe with some added focus from the highest levels of the company we can accelerate what Jimmy started

Ted Leonsis, America Online vice president

Mr Pitmann himself resigned this summer, just months after being appointed to the online division in a bid to revive the business.

The business includes Moviefone, Netscape and CompuServe but has failed to boost its flagging subscriber numbers and advertising income in the last year as rivals such as Microsoft continue to gain market share.

Mr de Castro, previously chief executive of AMFM Radio Group, was credited with helping to launch the latest version of the internet service, AOL 8.0.

But with the launch completed, he reportedly said he was "now looking for an opportunity to run a company".

Latest exit

Other executives who have departed AOL in the last few months include Mayo Stuntz, responsible for new initiatives across different divisions, and Myer Berlow, head of sales.

Mr de Castro's role will be filled by the vice president of America Online, Ted Leonsis.

Mr Leonsis, who ran the online brand between 1993 and 1996, said:

"I have done every job in the company. Maybe with some added focus from the highest levels of the company we can accelerate what Jimmy started."