Softbank, the web corporation woven by Masayoshi Son, appeared to be unravelling yesterday with the partial sale of its most prized asset.

The Tokyo-based holding company - which is one of the world's biggest e-investors - announced that it had sold 3.1m shares in the American internet portal Yahoo! for a $336m (£225m) profit.

Although the shares represent only 2.5% of Softbank's total stake in Yahoo!, the unloading of stock in one of the few undisputed money-spinners in the company's portfolio raises fresh doubts about Mr Son's ability to generate sufficient revenues to keep his business on an even keel.

Largely thanks to the strength of its holdings in Yahoo!, Softbank has been able to invest in more than 400 internet start-up companies around the world.

By nurturing these young businesses Mr Son has said his aim is to build an internet corporation that will soar in value when the firms are ready for public stock flotations.

"Our company is a bet on the internet," he has boasted.

This strategy helped Soft bank's stock soar twentyfold in the 12 months until February, when Mr Son appeared ready to overtake Bill Gates as the world's richest man.

The worldwide collapse of the internet bubble, however, has hit the Japanese-Korean tycoon harder than most, and his empire is now contracting fast. Yesterday, Softbank's stock price was ¥17,200, less than a tenth of its peak. Its profits last year were 70% down and it is believed to have been forced to sell off assets to stay liquid.

But there is a limit to how quickly Softbank can unwind.

The company has promised to hold on to many of its shares for six months. A further brake is that overseas sales would result in punishing taxes of up to 60%.

In another sign that Mr Son is scaling down his ambitions, Softbank appears to be pulling back from a proposed buy-out of the Nippon Credit Bank.

On Wednesday the government said it would no longer give a Softbank-led consortium exclusive rights to negotiate for the nationalised bank. The two sides had failed to hammer out a deal despite an extended deadline.

Mr Son - whose Korean background has always made him something of an outsider in Japan's conservative business world - is said to have hoped the purchase of such an institution would boost his prestige among Japan's business establishment.

He intended to convert Nippon Credit into an internet bank that would be positioned at the heart of his conglomerate and offer seed funding to start-up companies.

With the government's latest announcement that dream is fading fast.

Yesterday evening Japanese newspapers were suggesting that survival was now Mr Son's priority: "Anxieties over possible collapse of Softbank," read one headline.

But Mr Son, who has ridden out several previous credit crunches, is far from finished because Softbank remains Yahoo!'s biggest shareholder.