Tokyo-based mobile phone company Softbank Corporation has reached a deal to acquire 70% of US wireless carrier Sprint Nextel for $20.1 billion.

The deal is the largest ever foreign acquisition by a Japanese company.

The deal was announced today at a joint news conference in Tokyo by Softbank president Masayoshi Son and Sprint chief executive Dan Hesse.

It has been given a green light by the boards of both companies, but still needs approval from Sprint shareholders and US regulators.

Softbank said the deal is expected to be completed by the middle of next year.

The deal will bring together the third biggest mobile carriers of both Japan and the US, underlining the growth ambitions of Softbank, which has made a series of acquisitions and investments over the last couple of decades, including Yahoo Japan, the Japan unit of British mobile company Vodafone and Alibaba Group, a Chinese e-commerce company.

Son said the investment made sense because the US is the world's biggest market in smartphones, and it was still growing.

The news of the deal, which leaked late last week, had spooked investors worried about such a big investment and sent Softbank shares lower by about a third in recent sessions. Shares of Sprint, in contrast, shot up in anticipation of the offer. Sprint has struggled in recent years to compete with Verizon and AT&T.

The company has $21 billion in long-term debt, and has launched a costly network restructuring and signed a long-term contract to buy $15.5 billion worth of iPhones from Apple over four years.

Son is looking abroad for new growth as the Japanese mobile market has been stagnant for years. Softbank has been an exception in racking up strong profit despite such stagnation, largely on the popularity of the iPhone.

Softbank was the first carrier to offer the iPhone in Japan, and was the only one in initial years. The iPhone has been such a hit in Japan it has shaped Softbank's brand image and helped it lure customers away from its two bigger rivals.

Son said earlier this year that he is determined Softbank will overtake NTT DoCoMo, the longtime number one. He said the advent of smartphones means that the US and Japanese markets have much in common, and the two companies could benefit and learn from each other.

Before today's deal, the biggest overseas acquisition by a Japanese company was Japan Tobacco purchase of Gallaher Group of the UK in 2007 for about $19 billion.