SoftBank said it would seek to raise more than $20bn with the initial public offering of its Japanese mobile unit, as it hunts for funds to fuel its nearly $100 bn investment machine.

The proposed IPO shows how SoftBank, which is planning to give up control of Sprint in the US, is cashing out of its slower-growing assets so chief executive Masayoshi Son can focus on a technology portfolio that already includes a stake in Uber Technologies.