The report comes after Chinese startup DeepSeek sparked panic this week with a powerful new chatbot developed at a fraction of the cost of its US competitors, dealing a blow to markets.

SoftBank and OpenAI are part of the Stargate drive announced by US President Donald Trump to invest up to $500 billion in artificial intelligence infrastructure in the United States.

The Japanese firm's mooted investment would come on top of its commitment of more than $15 billion to Stargate, the FT said, citing people with direct knowledge of the negotiations.

"Ultimately, the Japanese company could spend more than $40bn on its partnership with OpenAI," the report said.

OpenAI will also invest $15 billion in Stargate, and SoftBank's equity investment in the US company could cover the latter's commitment to the AI infrastructure project, the FT added.

SoftBank was not immediately available for comment when contacted by AFP. Shares in the firm were down one percent in Tokyo trading Thursday morning.

The company, founded by Japanese tycoon Masayoshi Son, made spectacularly successful early bets on Yahoo! and Alibaba in the 1990s but some of its other investments have bombed.

Securing huge funding from Saudi Arabia, Abu Dhabi and others, Son -- an early backer of Trump -- has sought to pivot into AI, helped by SoftBank's stake in chip designer Arm.

Elon Musk has openly poured scorn on Stargate, saying on X this month that the main investors "don't actually have the money".

The world's richest man was an early investor in OpenAI and has long been in a feud with its founder Sam Altman, who called the Tesla founder's comments "wrong".

burs-stu/kaf/dan